Compensated color management systems and methods

ABSTRACT

Beamsplitters are used in projectors with retarder stack filters to orthogonally polarize primary colors, converting polarizing beamsplitters to color splitters and combiners. Geometric polarization rotations induced by beamsplitters at moderate f-numbers significantly degrading performance. Because retarder stacks may rely on a specific input polarization to perform properly, such skew rays result in color cross-talk. Retarder stacks according to the present invention are sensitive to the symmetries between input and output polarizer configurations. These stacks provide the polarization transformations that will compensate for skew rays, such that normal incidence performance is maintained for all incident light. Additionally, systems that utilize color selective polarization filters (CSP) and polarizing beam splitting elements such that an output analyzer is not required are disclosed. The CSP may be included in the above system. One exemplary CPS architecture includes two CSPs, a single polarizing beam splitting element combination for splitting and combining light between two of the three panels, and an output polarizing beam splitting element used as an analyzer such that a single CSP in the projection path. A second architecture uses an output combining chromatic polarizing beam splitting. A third architecture uses chromatic sheet polarizers to enhance CSP performance. In all of the architectures, an output CSP and clean up polarizer directly in front of the projection optics need not be included, thereby increasing transmission, improving imaging crispness by increasing the optical phase flatness of the projected light, and reducing cost.

[0001] This application is a continuation-in-part of U.S. application Ser. No. 10/000,227, filed Nov. 30, 2001, and is a continuation-in-part of provisional U.S. application Ser. No. 60/389,565, filed Jun. 18, 2002. The disclosures of these two applications are hereby incorporated by reference.

FIELD OF THE INVENTION

[0002] The present invention relates generally to color management in projection displays and similar systems, and more particularly to skew-ray correction in the color management of video projection systems. The present invention also relates generally to color management architectures for projection systems that utilize color selective polarization filters and polarizing beam splitting elements in projection displays and similar systems. The present invention further relates generally to color management in projection displays and similar systems that care combined with color management architectures for projection systems that utilize color selective polarization filters and polarizing beam splitting elements in projection displays and similar systems.

BACKGROUND

[0003] It is generally desirable for projection systems to produce high quality images while being compact and inexpensive. In prior art transmissive systems using polysilicon liquid crystal panels, high contrast is achieved by situating each panel between crossed sheet polarizers. Color management beamsplitters/combiners and other optics are external to these units, insuring that polarization is not contaminated and contrast degraded.

[0004] In projection systems using reflective panels, such as liquid crystal on silicon (LCOS) displays, beam routing is frequently accomplished using polarizing beamsplitters (PBSs). PBSs are four-port devices, comprising an input port, an output port, and a reflective panel port, with the fourth port typically unused. The behavior of thin-film PBS cubes, for instance, is such that s-polarized light is reflected, while orthogonally polarized light is substantially transmitted.

[0005] Efficient use of ultra-high pressure mercury (UHP) lamp emission to achieve high-brightness generally requires that the projection system operate at f2.8 or lower. F-number reduction to achieve high brightness is often accompanied by diminished contrast ratio. In practice, precise separation of input and modulated light becomes challenging when PBSs are used in even modest f-number systems. The control of skew rays in such systems has previously been analyzed in the context of contrast ratio preservation. A simple quarter-wave retarder between the panel and PBS is a well-known compensation scheme for managing contrast loss due to the PBS geometrical rotation.

[0006] Compact LCOS-based projection systems can be realized by combining the beam separation function of the PBS with a polarization based color management. Retarder stacks, as described in U.S. Pat. No. 5,751,384, enable encoding of color by polarization, thereby expanding the role of neutral PBSs to function as color splitters and combiners. However, such color management systems have the same skew-ray susceptibility found in the panel ports. In order to maintain high color quality and contrast in low f-number systems, the polarization encoding incorporates skew ray correction.

[0007] Color management architectures attempt to achieve high contrast, high throughput and good color saturation at a low cost. However, increasing one of the contrast, the throughput or the color saturation often results in a trade-off with one of the other properties. Avoid or minimizing this trade-off often requires additional or more expensive components which increases the cost of the architecture. Accordingly, there is a strong need in the art for color management architectures which avoid these and other trade-offs at a low cost.

SUMMARY OF THE INVENTION

[0008] An aspect of the invention is to provide an optical filter including at least three retarders where the at least three retarders cause optical rotation to light of a first spectrum substantially without introducing retardation.

[0009] An aspect of the invention is to provide an optical filter including at least three retarders wherein the at least three retarders act as a half wave plate to light of a first spectrum and wherein the at least three retarders act as a half wave plate with a π/4 displaced optical axis to light of a second spectrum.

[0010] An aspect of the invention is to provide an optical filtering method, the steps of the method including optically rotating light of a first spectrum without introducing retardation.

[0011] An aspect of the invention is to provide an optical filter method, the steps of the method including retarding light of a first spectrum and retarding light of a second spectrum. A half wave of retardation is provided by the retarding light of a first spectrum and a retardation equivalent to a half wave plate with a π/4 displaced optical axis is provided by the retarding light of a second spectrum.

[0012] An aspect of the invention is to provide an optical arrangement including a half waveplate and a pair of beam splitters oriented orthogonally to each other. The pair of beam splitters sandwiching the half waveplate.

[0013] An aspect of the invention is to provide a method of filtering light, steps of the method including splitting a first light beam to form a second light beam, retarding the second light beam with a half wave of retardation to form a third light beam, and splitting the third light beam.

[0014] An aspect of the invention is to provide an optical arrangement including a planar polarizer, a beam splitter, and an out-of-plane retarder between the planar polarizer and the beam splitter.

[0015] An aspect of the invention is to provide a method of filtering light, the steps of the method including polarizing an incident light beam to form a polarized light beam, retarding the polarized light beam with an out-of-plane retarder to form a retarded light beam and splitting the retarded light beam.

[0016] An aspect of the invention is to provide a compensating color selective polarization filter (CCSPF) which, substantially independent of input polarization angle, has no effect on the state of polarization (SOP) of one portion of the spectrum, while another portion of the spectrum is converted to the orthogonal SOP.

[0017] Another aspect of the invention is to provide a CCSPF which reflects the SOP of one portion of the spectrum about an axis, while another portion of the spectrum is reflected about an axis displaced by ±π/4 with respect to the aforementioned axis.

[0018] Another aspect of the invention is to provide a CCSPF which includes an out-of-plane uniaxial compensator.

[0019] Another aspect of the invention is to provide a skew ray compensated structure, including a first beam splitter, a second beam splitter, and a CCSPF. The CCSPF is between the first and second beam splitters, such that the SOP of skew rays transmitted by the first beam splitter are preconditioned so as to compensate for the skew-ray polarizing properties of the second beam splitter.

[0020] Another aspect of the invention is to provide a projection system including at least one structure comprising of a pair of beamsplitters bounding a CCSPF.

[0021] Another aspect of the invention is to provide a projection system including a pre-polarizer adjacent to a first CSPF that converts one spectrum of light to the orthogonal SOP, and does not change the SOP of another spectrum of light, a first PBS adjacent to the first CSPF for separating the one spectrum of light from the another spectrum of light, a second PBS adjacent to the first PBS, and a first display panel for selectively reflecting light adjacent to the second PBS, a first CCSPF between the first PBS and a third PBS that reflects the SOP of a first portion of the another spectrum of light about an axis and reflects the SOP of a second part of the another spectrum of light about another axis, a second panel for selectively reflecting light adjacent to a face of the third PBS opposing the first CCSPF, a third panel for selectively reflecting light adjacent to another face of the third PBS, a second CCSPF adjacent to a face opposing the third panel, the second CCSPF reflecting the SOP of the first portion of the another spectrum of light about an axis and reflecting the SOP of the second part of the another spectrum of light about another axis, a first half wave plate between the second PBS and the fourth PBS, a second CSPF that converts the SOP of one spectrum of light to the orthogonal state and does not effect the another spectrum of light adjacent to the fourth polarizing beam splitter, and a polarizer adjacent to the second CSPF.

[0022] An aspect of the invention is to provide a color management architecture including first reflective panel that modulates light of a first spectrum, a second reflective panel that modulates light of a second spectrum, a third reflective panel the modulates light of a third spectrum and at least four polarizing beam splitting elements. The light of the first and second spectrum is transmitted through more of the at least four polarizing beam splitting elements than the light of the third spectrum. The light source intensity or throughput efficiency of light of the third spectrum being a lower than the light source intensity or throughput efficiency of light of the first or second spectrums when throughput absent the losses associated with transmission or reflection through the polarizing beam splitting elements is excluded. A further aspect of this invention further including the absence of an output analyzer.

[0023] Another aspect of the invention is to provide a method of providing a color management including modulating a first spectrum of light with a first reflective panel, modulating a second spectrum of light with a second reflective panel and modulating a third spectrum of light with a third reflective panel. The light of the first and second spectrum is transmitted through more of at least four polarizing beam splitting elements than the light of the third spectrum. The throughput efficiency of light of third spectrum being a lower than the throughput efficiency of light of the first or second spectrums when throughput absent the losses associated with transmission or reflection through the polarizing beam splitting elements is excluded.

[0024] Another aspect of the invention is to provide a color management architecture including a first reflective panel that modulates a first spectrum of light and a second reflective panel that modulates a second spectrum of light adjacent a first polarizing beam splitting element, a third reflective panel that modulates a third spectrum of light adjacent a second polarizing beam splitting element, a chromatic polarizing beam splitting element. The first and second polarizing beam splitting elements reflecting light of a first polarization and transmitting light orthogonal to the first polarization. The chromatic polarizing beam splitting element transmitting light of a first polarization of the first spectrum, and reflecting light orthogonal to the first polarization. A further aspect of this embodiment including the absence of a polarization rotation element between the chromatic polarizing beam splitting element 1402. The first polarizing beam splitting element, polarizing beam splitting element and the chromatic polarizing beam splitting element having reflective surfaces oriented orthogonal to each other.

[0025] Another aspect of the invention is to provide a display architecture including an input polarizing beam splitter, a dichroic beam splitter that receives light including light of a first spectrum, a second spectrum and a third spectrum from the input polarizing beam splitter, a first polarizing beam splitter that receives light including the first spectrum from the dichroic beam splitter, a second polarizing beam splitter that receives light including the second spectrum and third spectrum from the dichroic beam splitter, a third polarizing beam splitter that receives light including the second spectrum and third spectrum from the dichroic beam splitter, a first panel for modulating light of the first spectrum adjacent the first polarizing beam splitter, a second panel for modulating light of the second spectrum adjacent the second polarizing beam splitter, and a third panel for modulating light of the third spectrum adjacent the second polarizing beam splitter. A reflective surface of the dichroic beam splitter is optically parallel to a reflective surface of the input polarizing beam splitter. A further aspect of this invention including a yellow/neutral filter, the input polarizing beam splitter being optically sandwiched between the dichroic beam splitter and the yellow/neutral filter. A still further aspect of this invention including a first dichroic filter that transmits light of the first spectrum optically between the dichroic beam splitter and the first polarizing beam splitter, and a second dichroic filter that transmits light of the second spectrum and the third spectrum optically between the dichroic beam splitter and the second polarizing beam splitter. A still further aspect of this invention including a first inorganic λ/2 plate optically between the dichroic beam splitter and the dichroic beam splitter, and a second inorganic λ/2 plate optically between the first polarizing beam splitter and the third polarizing beam splitter. The first inorganic plate and the second inorganic plate may be made of quartz. A still further aspect of this invention including a first polarization rotation filter that rotates the second spectrum of light but does not substantially rotate the third spectrum of light, and a second polarization rotation filter that rotates the third spectrum of light but does not substantially rotate the second spectrum of light. A still further aspect of this invention including the first polarization rotation filter and the second polarization rotation filter are retarder stacks.

[0026] Another aspect of the invention is to provide a display architecture including an input polarizing beam splitter, a dichroic beam splitter that receives light including light of a first spectrum, a second spectrum and a third spectrum from the input polarizing beam splitter, a first polarizing beam splitter that receives light including the first spectrum from the dichroic beam splitter, a second polarizing beam splitter that receives light including the second spectrum and third spectrum from the dichroic beam splitter, a third polarizing beam splitter that receives light including the second spectrum and third spectrum from the dichroic beam splitter, a first panel for modulating light of the first spectrum adjacent the first polarizing beam splitter, a second panel for modulating light of the second spectrum adjacent the second polarizing beam splitter, a third panel for modulating light of the third spectrum adjacent the second polarizing beam splitter, a first dichroic filter that transmits light of the first spectrum optically between the dichroic beam splitter and the first polarizing beam splitter, a second dichroic filter that transmits light of the second spectrum and the third spectrum optically between the dichroic beam splitter and the second polarizing beam splitter, a first inorganic λ/2 plate optically between the dichroic beam splitter and the dichroic beam splitter, a second inorganic λ/2 plate optically between the first polarizing beam splitter and the third polarizing beam splitter, a first polarization rotation filter that rotates the second spectrum of light but does not substantially rotate the third spectrum of light, and a second polarization rotation filter that rotates the third spectrum of light but does not substantially rotate the second spectrum of light. A reflective surface of the dichroic beam splitter is optically parallel to a reflective surface of the input polarizing beam splitter. A still further aspect of this invention including a yellow/neutral filter, the input polarizing beam splitter being optically sandwiched between the dichroic beam splitter and the yellow/neutral filter.

[0027] Another aspect of the invention is to provide a display architecture including an input wire grid polarizer optically followed by an o-plate, a dichroic beam splitter that receives light including light of a first spectrum, a second spectrum and a third spectrum from the input wire grid polarizer and the o-plate, a first polarizing beam splitter that receives light including the first spectrum from the dichroic beam splitter, a second polarizing beam splitter that receives light including the second spectrum and third spectrum from the dichroic beam splitter, a third polarizing beam splitter that receives light including the second spectrum and third spectrum from the dichroic beam splitter, a first panel for modulating light of the first spectrum adjacent the first polarizing beam splitter, a second panel for modulating light of the second spectrum adjacent the second polarizing beam splitter, a third panel for modulating light of the third spectrum adjacent the second polarizing beam splitter, a first dichroic filter that transmits light of the first spectrum optically between the dichroic beam splitter and the first polarizing beam splitter, a second dichroic filter that transmits light of the second spectrum and the third spectrum optically between the dichroic beam splitter and the second polarizing beam splitter, a first inorganic λ/2 plate optically between the dichroic beam splitter and the dichroic beam splitter, a second inorganic λ/2 plate optically between the first polarizing beam splitter and the third polarizing beam splitter, a first polarization rotation filter that rotates the second spectrum of light but does not substantially rotate the third spectrum of light, and a second polarization rotation filter that rotates the third spectrum of light but does not substantially rotate the second spectrum of light.

[0028] An aspect of the invention is to provide a method of projecting light including polarizing light with an input polarizing beam splitter, splitting light received from the input polarizing beam splitter into a first portion and a second portion with a dichroic beam splitter having a reflective surface optically parallel to a reflective surface of the input polarizing beam splitter, directing the first portion of light to a first panel with a first polarizing beam splitter, the first panel modulating and reflecting the first portion of light, splitting the second portion of light into a third portion of light and a fourth portion of light with a second polarizing beam splitter, the third portion of light being directed to a second panel and the fourth portion of light being directed to a third panel, the second panel modulating and reflecting the third portion of light, the third panel modulating and reflecting the fourth portion of light, combining modulated and reflected third and portions of light with the second polarizing beam splitter to form a fifth portion of light, and combining modulated and reflected first portion light with the fifth portion of light with a third polarizing beam splitter.

BRIEF DESCRIPTION OF THE DRAWINGS

[0029]FIG. 1 shows a prior art rotation intolerant Red/Blue filter spectrum viewed through parallel polarizers at normal incidence where the solid trace shows input/output light along the design axis and the dashed trace shows input/output light at 10° to the design axis;

[0030]FIG. 2 shows the spectrum of a prior art rotation intolerant Red/Blue filter between paired polarization beam splitters where azimuth averaged output spectra are plotted for half-cone angles of 0°, 5°, 10°, 15° and 20°, with the latter giving the highest side-lobe leakage;

[0031]FIG. 3 shows the azimuth averaged output spectrum of a prior art Green/Magenta rotation intolerant filter between paired polarization beam splitters plotted for illumination half-cone angles of 0°, 5°, 10°, 15° and 20°, with the latter giving the highest side-lobe leakage;

[0032]FIG. 4 is an illustration showing that skew rays have their polarization rotated, while normal and in-plane rays do not have their polarizations rotated;

[0033]FIG. 5 shows the crossed polarizer spectrum of 50:50 duty ratio stack using 600 nm film (505 nm 50% point) and 0.005% side lobes;

[0034]FIG. 6 shows the crossed polarizer spectrum of 50:50 duty ratio stack using 600 nm film (505 nm 50% point) and 0.005% side lobes, rotated by π/4;

[0035]FIG. 7 shows the crossed polarizer spectrum of 50:50 duty ratio stack using 600 nm film (505 nm 50% point) and 0.005% side lobes, rotated by π/4, with additional 600 nm film at π/2;

[0036]FIG. 8 shows the maximum side-lobe leakage with the stack oriented at π/4 for the designs of Table 2;

[0037]FIG. 9 shows a rotation tolerant Red/Blue filter spectrum viewed through parallel polarizers using an exemplary embodiment of the present invention where the solid trace shows input/output light polarized and analyzed parallel to the design axes and the dashed trace shows input/output light polarized and analyzed at 10° to the design axes;

[0038]FIG. 10 shows the spectrum of a rotation tolerant Red/Blue filter with the addition of a half-wave plate between paired polarization beam splitters with orthogonal faces where outputs are uniformly averaged over illumination half-cone angles of 0°, 5°, 10°, 15° and 20°, with the latter giving the highest side-lobe leakage;

[0039]FIG. 11 shows a skew-ray corrected structure, including a half-wave retarder CCSPF with wavelength selective optic axis orientation between PBSs with crossed faces;

[0040]FIG. 12 shows a skew-ray corrected structure, including a purely rotative CCSPF between PBSs with parallel faces;

[0041]FIG. 13 shows the spectrum of a Green/Magenta rotationally tolerant filter between paired polarization beam splitters uniformly averaged over half cone angles of 0°, 5°, 10°, 15° and 20° according to another exemplary embodiment of the present invention;

[0042]FIG. 14 shows a graph of θ as a function of φ;

[0043]FIG. 15 shows an out-of-plane uniaxial stack used to compensate for skew ray depolarization between a sheet polarizer and a PBS;

[0044]FIG. 16 shows a COLORQUAD™ color management system using CCSPFs and polarizing beamsplitters;

[0045]FIG. 17 shows another exemplary COLORQUAD™ color management system using CCSPFs;

[0046]FIG. 18 shows a 90° COLORQUAD™ color management system using CCSPFs and PBSs;

[0047]FIG. 19 shows a three-panel color projection system including skew-ray depolarization compensation;

[0048]FIG. 20 shows a two-panel exemplary color projection system including skew ray depolarization compensation;

[0049]FIG. 21 shows another two-panel exemplary color projection system including skew ray depolarization compensation;

[0050]FIG. 23 shows a single-panel projection system including skew-ray depolarization compensation;

[0051]FIG. 24 illustrates a projection device according to a first exemplary embodiment of the invention;

[0052]FIG. 25 illustrates an exemplary embodiment of FIG. 24 where blue light is directed to the first reflective panel;

[0053]FIG. 26 illustrates a projection device that uses unpolarized input light;

[0054]FIG. 27 illustrates a projection device that includes a dichroic input beam splitter;

[0055]FIG. 28 illustrates a projection device with a chromatic polarizing beam splitting element;

[0056]FIG. 29 illustrates a projection system with internal polarizers according to a first exemplary embodiment;

[0057]FIG. 30 illustrates a projection system with internal polarizers according to a second exemplary embodiment;

[0058]FIG. 31 illustrates a projection system with internal polarizers according to a third exemplary embodiment;

[0059]FIG. 32 illustrates a projection system with internal polarizers according to a fourth exemplary embodiment;

[0060]FIG. 33 illustrates a projection system with internal polarizers according to a fifth exemplary embodiment;

[0061]FIG. 34 illustrates another exemplary system according to the present invention;

[0062]FIG. 35 illustrate a spectra of an exemplary double notch filter;

[0063]FIG. 36 illustrates a spectra of an exemplary filter set used in FIG. 34;

[0064]FIG. 37 illustrates the transmission spectra of the blue and yellow transmitting dichroic plates;

[0065]FIG. 38 illustrates the and red transmissions of a FIG. 34 system compared to a FIG. 25 system; and

[0066]FIG. 39 illustrates another exemplary system having a red and green reflecting dichroic beam splitter.

DETAILED DESCRIPTION

[0067]FIG. 1 shows the spectrum of a prior art rotation intolerant Red/Blue CSPF viewed through parallel polarizers at normal incidence. The solid trace shows the spectrum with the input/output SOP along the design axis and the dashed trace shows the spectrum with the input/output SOP at 10° to the design axis. FIG. 2 shows the spectrum of a prior art rotation intolerant Red/Blue filter between paired polarization beam splitters. The output represents the average of spectra corresponding to rays distributed in azimuth with half-cone angles of 0°, 5°, 10°, 15° and 20°, with the latter giving the highest side-lobe leakage. FIG. 3 shows the spectrum of a prior art Green/Magenta rotation intolerant filter between paired polarization beam splitters. The output represents the average of spectra corresponding to rays distributed in azimuth with half-cone angles of 0°, 5°, 1°°, 15° and 20°, with the latter giving the highest side-lobe leakage.

[0068]FIGS. 1, 2 and 3 each demonstrate that color cross-talk occurs in prior art CSPFs as a consequence of skew ray depolarization. The cross-talk is manifested as side-lobes that reduce brightness and/or contrast. The side-lobes are independent of leakage along the design axis and are the consequence of compound retardation that can be a significant fraction of the total retardation of the stack. As such, light rays introduced parallel or perpendicular to the design axis can have low stop-band light leakage, while light rays that are rotated such that they are not parallel or perpendicular to the design axis often have substantial side-lobes that reduce the overall performance. Thus, color management systems using prior art CSPFs can have unacceptable contrast and/or brightness due to the presence of rotated polarization.

[0069] As FIG. 4 shows, skew rays travelling through a beam splitter have rotated polarization (relative to the axis defined by normally incident light), while in-plane rays have non-rotated polarization. In particular, light incident on the 45° reflecting dichroic coated surface of a PBS is polarized or analyzed along axes defined by the incident ray angle and the reflecting plane normal. For skew rays, this polarization direction is rotated relative to the axis defined by normally incident light.

[0070] The performance of color management systems may be improved by matching the CSPF design to the characteristics of polarizer/beamsplitter configurations, representing the more constrained subset of compensated CSPFs, or CCSPFs. The CCSPFs of the present invention provide the desired color selective properties, and also exploit input/output symmetry to compensate for skew ray depolarization. This results in improved performance, particularly at low system f-number.

[0071] An exemplary first method to correct for the above depolarization is to introduce CCSPFs based on stacked retardation films or plates. When compensating for the skew ray effects introduced by pairs of beamsplitters, a compensation scheme based on retarders with in-plane optic axes (or a-plates) is preferred. The polarization properties of such stacks are essentially independent of the incident angle of the light, particularly for biaxial films with Nz=0.5 (Nz(nx−nz)/(nx−ny) where nx is the refractive index in the x direction, ny is the refractive index in the y direction and nz is the refractive index in the z direction), and as such the polarization property of any one stack can be described mathematically by a Jones' Matrix which is independent of incident angle. A second method compensates for the geometric effects of a PBS used in combination with plane polarizers using one or more out-of-plane uniaxial birefringent layers (or o-plates).

[0072] Mathematically, the polarization axes (s and p) depend on the incident ray vector k (within the PBS) and the vector n, normal to the reflecting surface by the following expressions: ${s = \frac{n \times k}{\left| {n \times k} \right|}};\quad {p = \frac{s \times k}{\left| {s \times k} \right|}}$

[0073] Defining a coordinate system as shown in FIG. 4 where the normally incident rays propagate along the x-axis, and the normal to the reflecting plane is in the x,y plane, the s polarization axes for each of the normally incident, in-plane and skew rays respectively are: $\begin{pmatrix} 0 \\ 0 \\ 1 \end{pmatrix},{\begin{pmatrix} 0 \\ 0 \\ 1 \end{pmatrix}\quad {and}\quad {\begin{pmatrix} 0 \\ {\sin (\theta)} \\ {\cos (\theta)} \end{pmatrix}.}}$

[0074] The normally incident and in-plane rays are unchanged from what is conventionally defined as the s-polarization axis of a PBS, i.e., the normally incidents (NIS) axis. For skew rays, the s-polarization axis is rotated by an angle equal to the ray angle θ from the optical transmission axis in the PBS. The sign of the rotation depends on the sign of the incident angle and the normal of the reflecting surface. As a result, sequential PBSs with orthogonal reflecting surfaces have polarization axes for skew rays that are not parallel. This can be corrected using a CCSPF with polarization reflection properties.

[0075] For sequential PBSs with parallel reflecting surfaces, skew rays see parallel polarization axes but are rotated from the axes seen by normally incident rays. For color projection systems that are optimized for orthogonal polarization axes aligned with the NIS axis, this results in a loss of system optical performance that can be corrected using a CCSPF with pure rotation properties.

[0076] Table 1 describes the desired polarization performance of the various CCSPF filters using Jones' matrix notation. Case 1 has isotropic behavior in the non-rotated portion of the spectrum over the range of angles introduced. In the rotated portion of the spectrum, a pure π/2 rotator is used over the range of angles introduced. When the hypotenuse faces are orthogonal, the non-rotated band of Case 2 has a polarization reflection, as accomplished by a neutral half-wave plate with its optic axis aligned along or orthogonal to the NIS axis. The desired behavior in the rotated portion of Case 2 is accomplished using a structure that provides a pure half-wave retardation with its optic axis at ±π/4 to the NIS axis in the plane normal to the optical transmission axis. TABLE 1 Non-rotated band Rotated band

[0077] Proper selection of a retarder stack design to achieve Case 1 or Case 2 behavior requires understanding the general behavior of such structures. In color management systems, it is generally desirable that the spectrum be precisely partitioned according to the red, green and blue portions of the light source emission. This can include stable polarizing properties over a significant range of wavelengths, such that one spectral portion transmitted by the CCSPF has a substantially constant SOP, while a second portion emerges with the orthogonal SOP. The aforementioned wavelength stability can be achieved by using multi-layer structures, often with as many as twenty layers, to achieve acceptable performance in projection systems.

[0078] As the number of layers is increased to address the spectral criteria, the total retardance of the stack can rise to well over 10,000 nm. The composite structure can be considered the combination of a compound retardation and rotation at each wavelength, as described by a Jones' matrix. While the network synthesis technique identifies numerous stack designs that provide the same power spectrum, each has a unique Jones' matrix. (The network synthesis technique (see Harris et al. (1964), J. Opt. Soc. Am. 54:1267, Ammann et al. (1966), J. Opt. Soc. Am. 56:1746, and Ammann (1966), J. Opt. Soc. Am. 56:943) is a procedure for determining the orientations of the N retarders, and the exit polarizer, to obtain the desired amplitudes of the (N+1) impulses.) A common characteristic is that a stack has a significant compound retardation with a fairly stable eigenpolarization along the direction of the design axis. This retardation is often characterized by fairly linear phase within the flat regions of the spectrum. Typically, the retardation in the converted band differs from that in the non-converted band. Nonlinear phase often exists in the transition band of the spectrum.

[0079] Stack designs may be sorted to identify those that are most manufacturable. Optimum use of material occurs when the angles are small relative to the web direction since the films are often cut from roll stock. These stacks exhibit high overall compound retardance, and therefore the greatest sensitivity to rotated input/output polarization. The tight clustering of angles produces accumulation of retardation, such that the compound retardance is often a significant fraction of the total stack retardance. Conversely, stacks with the lowest compound retardation have a broad distribution of stack angles.

[0080] Case 1 represents a CCSPF that behaves isotropically in the non-converted band, and as a pure rotator in the converted band. The stack essentially vanishes in a range of wavelengths about the full-wave retardance in the non-converted band. As such, the compound retardation is near zero within this band. Pure rotation signifies that the stack also exhibits zero retardation within the converted portion of the spectrum. Such a structure has circular eigenpolarizations, in general transforming an input linear state to an output linear state with the prescribed rotation angle.

[0081] Consider a rotator formed from a pair of half-wave plates at arbitrary angles α and β. At the design wavelength, the Jones' matrix for the structure shows that the combination of two polarization reflections yields a pure rotation ${\begin{pmatrix} {\cos \quad 2\beta} & {\sin \quad 2\beta} \\ {\sin \quad 2\beta} & {{- \cos}\quad 2\quad \beta} \end{pmatrix}\begin{pmatrix} {\cos \quad 2\quad \alpha} & {\sin \quad 2\quad \alpha} \\ {\sin \quad 2\quad \alpha} & {{- \cos}\quad 2\quad \alpha} \end{pmatrix}} = {\begin{pmatrix} {\cos \left\lbrack {2\left( {\alpha - \beta} \right)} \right\rbrack} & {\sin \left\lbrack {2\left( {\alpha - \beta} \right)} \right\rbrack} \\ {- {\sin \left\lbrack {2\left( {\alpha - \beta} \right)} \right\rbrack}} & {\cos \left\lbrack {2\left( {\alpha - \beta} \right)} \right\rbrack} \end{pmatrix}.}$

[0082] When the angle between the half-wave plates is π/4, the matrix yields the desired pure rotator or $\begin{pmatrix} 0 & 1 \\ {- 1} & 0 \end{pmatrix}.$

[0083] Such behavior is not maintained, however, with small excursions from the half-wave retardance. Moreover, the insensitivity to absolute orientation of the structure experienced at the half-wave retardance, is not preserved at other wavelengths. As the number of layers is increased to achieve improved spectral performance, this sensitivity is typically enhanced.

[0084] If one considers only behavior at the half-wave wavelength, it can be shown using Jones' calculus that an odd number of films represents a reflection about an axis, while an even number of films produces a rotation. While this behavior may be confined to a very narrow portion of the converted band, it nonetheless gives some guidance in selecting number of films.

[0085] In general, the network synthesis method does not identify structures that provide zero compound retardation. Evaluating the Jones' matrix of each design can identify structures with the lowest compound retardation. Further reduction in compound retardation can be accomplished by placing a bias retarder parallel/perpendicular to the design axis according to the present invention. The bias retarder is crossed with the compound retarder of the stack, with retardation selected to minimize the net retardance. This is done with no impact on the spectrum along the design axis. The bias retarder can either proceed or follow the stack, with unique results in each case. Moreover, a different bias retardance on either side of the stack may be used to optimize results. If the bias retarder(s) can be well matched to the stack, then the structure satisfies that Case 1 criterion.

[0086] To satisfy Case 2 criteria, a structure that behaves like a pure half-wave retarder is desired, with a wavelength selective optic axis orientation. The optic axis is at 0 or π/2 throughout the non-converted portion of the spectrum, and has a ±π/4 orientation throughout the converted portion. The design of such a stack can be accomplished using the NST using a low retardation building block, or relatively high sampling frequency. This helps to insure the spectral coverage of the retardation requirement. Moreover, this methodology has the potential to build an achromatic half-wave retardance into the design without additional films. To avoid a large number of laminations with high sampling frequency, designs that can be reduced via film pairs can be identified.

[0087] Such structures can alternatively be designed in a modular form, including the aforementioned isotropic/optically active structure in combination with a neutral half-wave retarder at 0 or π/2. In this case, the difference between the bias retarder and the compound retardance is a half-wave. Mathematically, the output in the non-converted band is ${{\begin{pmatrix} {- 1} & 0 \\ 0 & 1 \end{pmatrix}\begin{pmatrix} 1 & 0 \\ 0 & 1 \end{pmatrix}} = \begin{pmatrix} {- 1} & 0 \\ 0 & 1 \end{pmatrix}},$

[0088] corresponding to the matrix for a half-wave retarder at 0. The output for the converted band neglecting absolute phase factors is, ${{\begin{pmatrix} {- 1} & 0 \\ 0 & 1 \end{pmatrix}\begin{pmatrix} 0 & 1 \\ {- 1} & 0 \end{pmatrix}} = \begin{pmatrix} 0 & 1 \\ 1 & 0 \end{pmatrix}},$

[0089] corresponding to the matrix for a half-wave retarder at π/4.

[0090] The modular approach separates the color selective rotation and polarization reflection criteria, making the latter a neutral operation. However, zero-order half-wave retarders may not be sufficiently neutral to span the entire spectral range. The half-wave center wavelength can be selected to optimize system level performance in this situation. Materials with low birefringence dispersion, or combinations of materials with different dispersions, can also be used to minimize this effect. Beyond this, compound structures, such as Pancharatnam half-wave plates, can be used that broaden the range of the half-wave retardance. In order to enjoy the benefits of broader half-wave bandwidth in practice, care must be taken to insure that the wavelength stability of the optic axis is preserved.

[0091] Another modular design splits the color selective portion of the stack, such that a half-wave retarder resides between matched color selective stacks. Consider an input stack that provides a pure π/4 rotation, an output stack that provides a pure −π/4 rotation, and a central 0-oriented neutral half-wave retarder. In the non-converted portion of the spectrum, the input/output stacks have substantially no effect on the state of polarization, yielding a 0-oriented half-wave retarder. In the converted portion of the spectrum the output is given by, ${{\frac{1}{\sqrt{2}}\begin{pmatrix} 1 & {- 1} \\ 1 & 1 \end{pmatrix}\begin{pmatrix} {- 1} & 0 \\ 0 & 1 \end{pmatrix}\frac{1}{\sqrt{2}}\begin{pmatrix} 1 & 1 \\ {- 1} & 1 \end{pmatrix}} = \begin{pmatrix} 0 & 1 \\ 1 & 0 \end{pmatrix}},$

[0092] corresponding to a half-wave retarder at π/4, as set forth in Case 2.

[0093] Consider the example of a CCSPF that acts as pure 90° rotator in one portion of the spectrum, and is isotropic in another portion of the spectrum. The structures generated using the NST are evaluated by observing their modeled spectra as they are effectively rotated between crossed polarizers. The figure of merit used is the side-lobe level at the worst-case orientation of 45-degrees, though the slope of the transition band may also be taken into consideration.

[0094] The example used is the 50:50 duty ratio case, which has the desired property that double-retardance films can be used to decrease the number of laminations. All but one of the films has a double-retardance in the 50:50 case. As such, the structures have the performance of an N-layer design, but reduce the number of films by a factor of (N−1)/2N. Provided that the retardance tolerance and Nz factor can be substantially maintained at the double-retardance value, and that a retardance value can be selected to achieve spectral requirements, the 50:50 case can be attractive. Alternatively, non-50:50 duty ratio cases can be considered.

[0095] For a particular value of N, all designs with (N−1)/2 double retardance values are extracted and evaluated according to their retardance along the design axis. This retardance generates oscillations in transmission with the stack oriented at π/4. Generally, stacks with a clustering of angles (typically near zero) have the highest composite retardation. The inclusion of additional films may provide zero net retardation. However, this can become impractical where the number of total number of films becomes large.

[0096] Conversely, designs with a wide distribution in angles can have very low composite retardation, which can be eliminated with the addition of one or two additional films. In the example given, the additional film is a single-retardation film at π/2. With the addition of this film, the polarization transformation is nearly insensitive to the orientation of the structure. In effect, the 0-orientation and π/4-orientation spectra are generated by different stack designs that have substantially the same polarization transformations. The performance at intermediate angles typically varies monotonically between the two extreme cases.

[0097] Consider the case of a 0.005% ripple 50:50 design, compensated with a single retardation film at π/2. A new design can be generated by rotating the structure by π/4, such that the external π/2-oriented film assumes the role of the π/4-oriented single film on the opposite side, which vanishes under this rotation. The two designs can be compared on a film-by-film basis by swapping this generated stack end-for-end.

[0098] For a particular N, the design with minimal sensitivity to rotation can be tracked as a function of side-lobe level at zero-orientation. In the case of N=15, the side-lobe level at π/4 fell as the zero-orientation side-lobe level was reduced. Table 2 shows an exemplary design and the changes in individual retarder angles as the side lobe levels are changed.

[0099] Note that the external single-retardance films do not change significantly in orientation (because of the 50:50 nature), but that the largest change in side lobe level (from 0.5% to 0.005%) is accomplished with more than 23-degrees of change in orientation of internal double-retardation films. These changes have a relatively large influence on the spectrum when the stack is rotated by π/4.

[0100] Consider the 0.005% design in Table 2. Table 3 shows the layout after rotating the design by π/4 and swapping the stack end-for-end. While this generates two distinct designs, their behavior has some similarities. The internal double-retardation structure is somewhat insensitive to sign change relative to the single-retardance film. That is, the magnitudes of the orientations of corresponding double-retardation films are similar, but the signs are reversed with respect to the single-retardation film.

[0101]FIG. 5 is the crossed-polarizer spectrum for the 50:50 design with 0.005% side lobes shown in Table 2 based on a 600 nm film. This embodiment includes a blue-yellow filter, such as described in U.S. Pat. Nos. 5,751,384, 5,990,996, 5,822,021, 5,953,083, 6,078,374, 6,046,786 and 5,658,490, each of which are incorporated herein by this reference. FIG. 6 is the stack design of FIG. 5 rotated by π/4 between crossed polarizers, showing that the first-order linear half-wave retardation in the blue nearly eliminates the blue pass-band. FIG. 7 shows the same stack design at π/4, with the addition of a single 600 nm film. As Table 2 shows, the additional film is at π/2, which is crossed with the compound retardance. FIG. 7 shows that this film is well matched to the compound redardance, resulting in a blue spectrum that is well matched to FIG. 5. The side lobe level is somewhat worse than the FIG. 5 spectrum (0.38%) and that the transition slope is also relatively shallow. FIG. 8 shows the maximum side-lobe leakage with the stack oriented at π/4 for the designs of Table 2. The largest leakage occurs for the 0.5% design and lowest leakage occurs for the 0.005% design. The side lobe level is measured along the design axis. TABLE 2 Eight-Layer (50:50 Duty-Ratio) Rotationally invariant Designs With Different Maximum Ripple Amplitudes Angle difference between 0.5% Retard- and 0.005% ance 0.5% 0.3% 0.1% 0.02% 0.005% designs  Γ 90.0° 90.0° 90.0° 90.0° 90.0°   0° 2Γ −33.7° −32.9° −30.9° −27.3° −22.9° 10.8° 2Γ −50.0° −49.3° −47.1° −41.8° −33.4° 16.6° 2Γ −24.6° −22.5° −17.9° −9.7° −1.0° 23.6° 2Γ −69.7° −69.7° −69.1° −66.2° −61.1°  8.6° 2Γ 31.8° 30.8° 30.0° 31.7° 36.4°  6.4° 2Γ 61.7° 63.3° 67.4° 75.6° 85.6° 23.9° 2Γ 46.9° 47.4° 49.0° 53.5° 61.1° 14.2°  Γ 46.0° 46.0° 46.2° 46.5° 46.7°  0.7°

[0102] TABLE 3 Comparison Of Rotationally Invariant Design With Transformed Design (Design generated from 45° rotation and reverse in order) Retardance 0.005% 0.005% after transformation  Γ 90.0° −1.7° 2Γ −22.9° 16.1° 2Γ −33.4° 40.6° 2Γ −1.0° −8.6° 2Γ −61.1° 73.9° 2Γ ˜36.4° −46.0° 2Γ 85.6° −78.4° 2Γ 61.1° −67.9°  Γ 46.7° 45.0°

[0103]FIG. 9 shows the spectrum of an exemplary Red/Blue CCSPF designed with rotation invariance according to the Case I criteria. This Red/Blue filter is viewed between parallel sheet polarizers with the design axis of the filter parallel and at 10° to the polarization axis. The performance of this filter is greatly improved as shown by the absence of side lobes under a 10° rotation, in comparison to that of FIG. 1.

[0104]FIG. 10 shows the spectra of such a skew ray tolerant filter comprising a rotationally invariant Red/Blue CCSPF with a 0-oriented zero-order half-wave retarder between paired PBSs with orthogonal faces. The output represents the uniform weighted azimuth-averaged spectrum for half-cone angles of 0°, 5°, 10°, 15° and 20°. The performance of this filter is greatly improved as shown by the absence of the side lobes in comparison to that of FIG. 2.

[0105]FIGS. 11 and 12 show skew ray corrected structures 100. FIG. 11 is substantially the same as FIG. 12 except that a first PBS 102 has an orthogonal orientation as compared to the first PBS 102 of FIG. 12 and output light beam having a first spectrum 104 and the output light beam having a second spectrum 106 have transposed exit points from the second PBS 108. FIG. 12 shows a skew-ray corrected structure, including a purely rotative CCSPF 110 between PBSs with parallel faces. Light 112 is input into a first PBS 102 and transmitted to the purely rotative CCSPF 10 that rotates the polarization orientation of a first part of the input light. The second PBS 108 then separates the input light into the first output light beam 104 and the second output light beam 106 according to its polarization. The input light that was not rotated being transmitted through the second PBS 108.

[0106]FIG. 11 shows a skew-ray corrected structure, including a half-wave retarder CCSPF 114 with wavelength selective optic axis orientation between PBSs with crossed faces. Light is input into a first PBS 102 that is rotated as compared to the first PBS 102 of FIG. 12. The light is then transmitted to a CCSPF 114 that acts as a half-wave retarder. The CCSPF 114 reflects the SOP of the received light. The second PBS 108 separates the light received from the CCSPF 114 into a first output light beam 104 and the second output light beam 106 based on the polarization of the light.

[0107]FIG. 14 shows a graph of optical axis angle 0 verse skew ray angle formed with the x-axis φ. As is shown, there is only a 0.5° variation for θ (φ) which means θ is nearly independent of φ.

[0108]FIG. 15 shows the spectra of a Green/Magenta rotationally tolerant filter between paired polarization beam splitters plotted for half cone angles of 0°, 5°, 10°, 15° and 20°. The performance of this filter is much improved as compared to that of FIG. 3 as shown by the absence of the side lobes.

[0109] The compensation scheme is modified when the CSPF is situated between polarizers with different symmetry properties. This occurs, for example when compensating for skew-ray depolarization in a CSPF that is sandwiched between a sheet polarizer and a PBS. If a PBS follows a CSPF preceded by a sheet polarizer, an out-of-plane compensator according to the present invention transforms the polarizations into the coordinates of the PBS.

[0110] Thus, a plane polarizer does not share the same symmetry as a PBS. The normally incident rays have the same input polarization axes as that of the skew rays, whereas the output polarization is analyzed with the axes rotated for the skew rays and not those normally incident. The skew and normal ray polarizations are acted upon differently.

[0111] This embodiment of the invention uses incident angle dependent polarization manipulation so that the skew rays see a different Jones' Matrix to those normally incident. This generally can be achieved by introducing birefringent elements that have axes with a component along the system optic axis, e.g. out of the plane of the film, such that the manipulation has comparable symmetry as that of a PBS.

[0112] Additionally, to achieve the odd type symmetry of the PBS in which the axes rotate opposite for rays above and below the system optic axes, the optic axis (or axes) should be in the plane (e.g., the x,y plane) defined by the system optic axis (e.g., the x axis) and the normal to the PBS reflecting plane (e.g., the line x−y=constant). Thus, a birefringent element or stack acts as a half-wave reflector to skew rays only and the reflection angle is proportional to the skew ray incident angle having the correct proportionality constant. One such exemplary embodiment is a single uniaxial birefringent stack with its retardance substantially constant or only varying by a relative small amount over small changes in angle. Thus, the half wave reflection property can be maintained over significant rotation of the skew ray incident angle.

[0113] From the second method, one can determine a uniaxial birefringent material whose optic axis n is at an angle θ from the y axis in the x,y plane as follows: $n = \begin{pmatrix} {\sin (\theta)} \\ {\cos \quad (\theta)} \\ 0 \end{pmatrix}$

[0114] The propagation vector k for a skew ray propagating in the x,z plane with angle φ from the x axis is: $k = \begin{pmatrix} {\cos \quad (\varphi)} \\ 0 \\ {\sin \quad (\varphi)} \end{pmatrix}$

[0115] The uniaxial optic axis as seen by the skew ray in the z,y plane can be determined from the vector cross product $\frac{n \times k}{\left| {n \times k} \right|}$

[0116] and the result projected onto the two dimensional y,z plane to give: $\quad\begin{pmatrix} {{- {\cos (\varphi)}} \cdot {\cos (\theta)}} \\ {{\sin (\varphi)}\quad {\sin (\theta)}} \end{pmatrix}$

[0117] This optic axis makes an angle with the y axis of: $= {a\quad {\cos \left\lbrack \frac{\left( {1\quad 0} \right)\begin{pmatrix} {{- {\cos (\varphi)}}\quad {\cos (\theta)}} \\ {{\sin (\varphi)} \cdot {\sin (\theta)}} \end{pmatrix}}{\begin{pmatrix} {{- {\cos (\varphi)}}\quad {\cos (\theta)}} \\ {{\sin (\varphi)}\quad {\sin (\theta)}} \end{pmatrix}} \right\rbrack}}$

[0118] If the retardance is a half wave then the polarization of the skew-ray is reflected about this axis, forming an angle of 2ρ with respect to the y axis. To mimic a PBS, 2ρ=φ resulting in the expression $\quad {{\cos \left( \frac{\varphi}{2} \right)} = \frac{\left( {1\quad 0} \right) \cdot \begin{pmatrix} {{- {\cos (\varphi)}}\quad {\cos (\theta)}} \\ {{\sin (\varphi)} \cdot {\sin (\theta)}} \end{pmatrix}}{\begin{pmatrix} {{- {\cos (\varphi)}} \cdot {\cos (\theta)}} \\ {{\sin (\varphi)}\quad {\sin (\theta)}} \end{pmatrix}}}$

[0119] This expression can be expanded for the small angles typical of projection systems (e.g., φ<15° in the PBS). As FIG. 14 shows, the optic axis angle θ is nearly independent of the input ray propagation angle φ. Thus, a uniaxial material with its optic axis at about 26.25° (±0.25°) to improve CSPF performance by compensating for depolarization. One stack design can be generally used in a projection system to compensate any number of filters between PBSs since the compensator has the added advantage of being broad band.

[0120] The stack retardance should be nearly a half-wave over the entire applicable spectrum of light. For visible light, a suitable design wavelength λ₀ would be 500 nm. However, this may be changed depending on the projection system in which the stack is to be installed. The extraordinary, ne, and ordinary, no, refractive indices and thickness, d, should be selected to adhere closely to the following equation ${\left( {\frac{{ne}\quad {no}}{\sqrt{{{no}^{2} \cdot {\cos (\theta)}^{2}} + {{ne}^{2} \cdot {\sin (\theta)}^{2}}}} - {no}} \right) \cdot d} = \frac{\lambda_{0}}{2}$

[0121] with θ being between about 26.0° to 26.5°. To make the out-of-plane compensator more achromatic, a further even number of standard in-plane uniaxial or biaxial half wave films can be made with compound retarder designs that assume the out-of-plane retarder acts as a simple in-plane half-waveplate. For any particular design, further optimization can be carried out by altering the out-of-plane orientations of the additional films to optimize the system performance.

[0122] Alternatively, the invention provides for correction of skew ray polarization axes between elements that are neither orthogonal nor parallel but at an intermediate angle. This is achieved by reducing the out-of-plane angle from 26.25°, such that near parallel planes can be made, so as to avoid mixing polarizations. The exact angle is determined according to the geometry of the system and the above equations. Additionally, correction of skew ray polarization axes between elements that are neither orthogonal nor parallel but at an intermediate angle could be implemented in conjunction with dichroic beam splitters cause depolarization through skew ray effects.

[0123]FIG. 15 is an exemplary embodiment for compensating structures that do not intrinsically posses symmetry, where an out-of-plane uniaxial stack is used to compensate for skew ray depolarization. The sub-system 200 includes a polarizer 202 such as a sheet polarizer, a CSPF 204, an out-of-plane compensator 206 and a PBS 208. Incident light 210 is polarized by the polarizer 202 and then is selectively rotated by the CCSPF 204. Light from the CCSPF 204 then passes through the out-of-plane compensator 206 to compensate for skew ray depolarization before entering the PBS 208. Light from the out-of-plane compensator 206 goes into the PBS 208 which separates it into a first output light beam 212 and a second output light beam 214.

[0124]FIG. 16 shows a low f-number COLORQUAD™ color management system 300. In the system 300, s-polarized white light 302 is incident on a first CSPF 304 that converts light of a first spectrum to the orthogonal polarization, that is subsequently transmitted through the first PBS 306. Light that is complementary to the first spectrum remains s-polarized and is subsequently reflected by the first PBS 306. Light having the first spectrum is then transmitted by a second PBS 308 and is subsequently incident upon a first reflective panel 310, which includes a quarter wave plate 312. In the ON-state, light reflected from the first reflective panel 310 is converted from p to s polarization, thereby enabling the second PBS 308 to reflect the light through a first 0-oriented half wave plate 314. Skew-ray corrected light is then reflected by a third PBS 316. The third PBS 316 reflects the first spectrum into a second CSPF 318, which converts light of the first spectrum to p-polarization, after which it is analyzed by a clean-up polarizer 320.

[0125] Incident light 302 of a second and third spectrum is reflected by the first PBS 306 onto a first CCSPF 322, which reflects the SOP of the second spectrum about a ±π/4 axis. Light of the second spectrum is transmitted through a fourth PBS 324 to a second reflective panel 326, which includes a quarter wave plate 328. The first CCSPF 322 reflects the SOP of the third spectrum about 0, and it is subsequently reflected by the fourth PBS 324 to a third reflective panel 330, which includes a quarter wave plate 332. ON-state light reflected from the second and third reflective panels 326, 330 returns to the PBS with the orthogonal state, so that light in the second and third spectra is incident on a second CCSPF 334. The second CCSPF 334 reflects the SOP of the second spectrum about a π/4 axis and reflects the SOP of the third spectrum about 0, thus restoring both spectra to p-polarization. Skew ray corrected p-polarized light is then transmitted by the third PBS 316, the second CSPF 318 and the clean-up polarizer 320. Situated between each panel and PBS is a quarter-wave plate oriented to correct for skew rays.

[0126] In practice, CSPF filters 304 and 318 are green/magenta retarder stacks with offset 50% spectral transition points. Notches can be introduced via absorption of true yellow and true cyan light that exists between the transitions of the CSPFs. This true yellow and true cyan light is substantially absorbed by the clean-up polarizer 320, producing an enhanced color gamut and increased contrast.

[0127] By eliminating green light from the spectrum reflected by the first PBS, 306, the transition between the second and third spectra can be relaxed (50% point in the green), with no consequence on system performance. This simplifies the stack design for the CCSPFs 322, 334. In the event that zero-order half-wave plates are used in combination with CSPFs that act as pure rotators to form the CCSPF, the center wavelength can be different for each filter. If the center wavelengths are offset to improve performance, preference may be given to centering the half-wave retardance in the third spectral band for the first CCSPF 322 and for centering the half-wave retardance in the second spectral band for the second CCSPF 334.

[0128]FIG. 17 shows another color projection system 400 which is similar to the system 300 of FIG. 16 but uses a pre-polarizing PBS 402 in combination with an input CCSPF 404 to provide an illumination path that is skew ray compensated. The CCSPF 404 functions as a pure 90° rotator in one portion of the spectrum and is isotropic in another portion of the spectrum. The pre-polarizing PBS 402 is advantageous due to its tolerance to high intensity illumination often seen at the light input point (e.g., near or at the light source) of color projection systems. An additional half-wave plate 406 is also incorporated to provide skew ray correction for light of the first spectrum in the illumination arm. An additional out-of-plane compensator (not shown) placed between the third PBS 816 and the clean up polarizer 324 can be used to compensate for skew-ray mismatch between those elements without significantly increasing the back focal length of the projection lens.

[0129]FIG. 18 shows a low f-number 90° COLORQUAD™ color management system 500. In the system 500, s-polarized white light 502 is incident on a first CSPF 504 that converts light of a first spectrum to the orthogonal polarization, that is subsequently transmitted through the first PBS 506. Light that is complementary to the first spectrum remains s-polarized and is subsequently reflected by the first PBS 506. Light having the first spectrum is then transmitted by a second PBS 508 and is subsequently incident upon a first reflective panel 510. In the ON-state, light reflected from the first reflective panel 510 is converted from p to s polarization, thereby enabling the second PBS 508 to reflect the light through a first π/4-oriented half wave plate 512. Skew-ray corrected light is then transmitted by a third PBS 514. The third PBS 514 transmits the first spectrum into a second CSPF 516, which converts light of the first spectrum to s-polarization light 518, after which the s-polarized light 518 is analyzed by a clean-up polarizer (e.g., an s-polarizer) (not shown). Quarter wave plates (not shown) can be inserted between each panel and the PBS. Also, the compensation methods shown in FIG. 17 can be used to improve the system performance.

[0130] Incident light 502 of a second and third spectrum is reflected by the first PBS 506 onto a first CCSPF 520, which reflects the SOP of the second spectrum about a ±π/4 axis. Light of the second spectrum is transmitted through a fourth PBS 522 to a second reflective panel 524. The first CCSPF 520 reflects the SOP of the third spectrum about 0, and it is subsequently reflected by the fourth PBS 522 to a third reflective panel 526. ON-state light reflected from the second and third reflective panels 524, 526 returns to the PBS 522 with the orthogonal state, so that light in the second and third spectra is incident on a second CCSPF 528. The second CCSPF 528 reflects the SOP of the third spectrum about a ±π/4 axis and reflects the SOP of the second spectrum about 0, thus restoring both spectra to s-polarization. Skew ray corrected s-polarized light is then reflected by the third PBS 514, the second CSPF 516 and the clean up polarizer (not shown). Situated between each panel and PBS is a quarter-wave plate oriented to correct for skew rays. Again, the compensation methods such as shown in FIG. 17 can be used to improve system performance.

[0131] Using a green-magenta CSPF for the first CSPF 504, light in the second and third spectra represents blue and red (or vice versa). Because most UHP lamps are green rich and red deficient, the 90° quad accomplishes an attractive color balance function. In order for green ON-state light to exit the system, it is transmitted through a PBS three times and is reflected by a PBS only once. Conversely, magenta light in the ON-state is reflected from a PBS three times and transmitted through a PBS once. Due to the relatively high efficiency of a PBS reflection, the magenta light exits the system with higher throughput than green light, thus color balancing the system.

[0132]FIG. 19 shows another three panel color management system 600 according to another exemplary embodiment the present invention. The system 600 includes a first CSPF of a first rotation spectrum 602, a PBS 604, a spacer cube 606, a first reflective color panel 608, a compensation element 610 such as a half-wave retarder, a dichroic beam splitter 612, a second reflective color panel 614, a third reflective panel 616, a second CSPF 618 and an output p-polarizer 620. The system 600 receives an input light 622 at the first CSPF 602 and outputs output light 624 at output p-polarizer 620. The dichroic reflecting surface of the dichroic beam splitter 612 is orthogonal to that of the PBS 604. The orthogonal relationship places the second reflective panel 614 on the side opposite the cube 606 and avoids mechanical interference between the second reflective panel 614 and the cube 606, simplifying assembly. The orthogonal reflecting surfaces are skew ray compensated by the compensation element 610, that is sufficiently achromatic. Additional compensation methods, such as illustrated in FIG. 17, can be used to improve system performance.

[0133]FIG. 20 shows a 2-panel exemplary color projection system 700 including skew ray depolarization compensation. This two panel embodiment is further described in patent application Ser. No. 09/779,443 which is incorporated herein by reference. The system 700 of FIG. 20 includes a first polarizing PBS 702, a blue/green ColorSwitch™ 704, a second PBS 706, a rotation tolerant cyan/red CCSPF 708, a third PBS 710, a red reflective panel 712, a blue/green reflective panel 714, a red/cyan CSPF 716 and an output p polarizer 718. Input light 720 is input into the system 700 at the first polarizing PBS 702. Light from the first polarizing PBS 702 is then incident upon the blue/green ColorSwitch™ 704. The blue/green ColorSwitch™ 704 alters the SOP of the light such that the second PBS 706 determine whether blue or green will be transmitted through the second PBS 706. A passive portion of the color switch converts red light to an orthogonal polarization, but the active portion of the color switch has no effect on the SOP of red light. The rotation tolerant cyan/red CCSPF 708 then receives the light from the second PBS 706 and changes the SOP of the blue/green light so that the third PBS 710 will direct the blue and green light to the blue/green reflective panel 714 and will direct the red light to the red reflective panel 712. After reflection from the panels 712, 714, red light is reflected by the third PBS 710 and blue/green light is transmitted by the third PBS 710 to the red/cyan CSPF 716 and the output p polarizer 718 to form an output light beam 722. An out-of-plane compensated CCSPF can be used to provide additional skew-ray compensation between the third PBS 710 and the output p polarizer 718.

[0134]FIG. 21 shows a 2-panel exemplary color projection system 800 including skew ray depolarization compensation. White light from a light source 802, which may be polarized, is incident on a dichroic mirror 804, such that blue and green are transmitted to an s-polarizer 806. The SOP of blue and green are actively controlled using color switch 808. The output is reflected by fold-mirror 810, and the polarization is subsequently analyzed by a PBS 812, such that the transmission of blue and green are temporally modulated onto an achromatic π/4-oriented half-wave retarder 814. Blue and green skew-ray corrected light converted to s-polarization by the half-wave retarder 814 is then reflected by a PBS 816, with ON-state light converted to p-polarization by a blue/green reflective panel 818. ON-state light transmitted by the PBS 816 is transmitted by red/cyan (R/C) CSPF 820 with no change in SOP, and the polarization is subsequently analyzed by a clean-up polarizer 822. Out-of-plane compensators (not shown) can be inserted between the sheet polarizers and PBSs to improve system performance.

[0135] Red light reflected by the dichroic mirror 804 is polarized by an s-polarizer 824 then reflected by a fold mirror 826 and reflected by a PBS 812. The half-wave retarder 814 converts red light to p-polarization, which is then transmitted through the PBS 816 to a red reflective panel 828. Light reflected from the red reflective panel 828 is the PBS 816 and is converted to p-polarization by the RIC CSPF 820. This p-polarized light is then analyzed by the clean-up polarizer 822. Again, additional compensation methods, such as is illustrated in FIG. 17, can be used to improve the performance of this system.

[0136]FIG. 22 shows a 2-panel exemplary color projection system 900 including skew ray depolarization compensation. FIG. 22 is similar to FIG. 21 except the half-wave retarder 814 has been replaced with an achromatic 90° rotator having no retardation 902 and the PBS 816 has been replaced with an orthogonal PBS 904. The achromatic rotator 902 rotates the polarization of the light passing through it whereas the half-wave retarder 814 provides reflection about an axis. The rotation instead of reflection of the light makes the orientation of the PBSs 812, 904 parallel instead of orthogonal and reverses the positions of the blue/green reflective panel 818 with that of the C/R CSPF 820 and the clean-up polarizer 822. The achromatic pure-rotator may be a simpler structure to manufacture and is less sensitive to orientation in the system. For example, a stack formed of polycarbonate films with 250 nm retardation and angles 90°, −18.7°, 3.9°, −37.6°, 31.5°, and 47.7°, efficiently rotates all visible light, nearly independent of its orientation with the input polarization.

[0137]FIG. 23 shows a 1-panel exemplary color projection system including skew ray depolarization compensation. This single panel embodiment without skew ray depolarization compensation is further described in patent application Ser. No. 09/126,330 which is incorporated herein by reference. The system 1000 of FIG. 23 includes a first polarizing PBS 1002, a red/green/blue ColorSwitch™ 1004, a second PBS 1006, a reflective panel 1008, an achromatic out-of-plane compensator 1010, and an output s-polarizer 1012. Input light 1014 is input into the system 1000 and polarized by the first polarizing PBS 1002 and transmitted to the ColorSwitch™ 1004. The light from the ColorSwitch™ 1004 travels through the second PBS 1006 and is incident upon the reflective panel 1008. The reflective panel 1008 reflects light back to the second PBS 1006, which reflects the light through the compensator 1010 and the output s-polarizer 1012. The light transmitted through the output s-polarizer 1012 is the output light 1016.

[0138] The present invention relates to color management architectures for three panel projection systems that utilize color selective polarization filters (CSP) and polarizing beam splitting elements. The present invention constitutes a distinct set of architectures in that an output analyzer may be omitted. One exemplary embodiment of the present invention includes two CSPs, a single polarizing beam splitting element combination for splitting and combining light between two of the three panels, and an output polarizing beam splitting element used as an analyzer. This embodiment provides a single CSP in the projection path. A second exemplary embodiment uses an output combining chromatic polarizing beam splitting. This embodiment avoids the need to include a CSP in any projection path. A third exemplary embodiment uses chromatic sheet polarizers to enhance CSP performance. In all cases, the output CSP and clean up polarizer directly in front of the projection optics need not be included, thereby increasing transmission, improving imaging crispness by increasing the optical phase flatness of the projected light, and reducing cost. These exemplary embodiments are discussed with reference to the figures where like reference numbers refer to like parts.

[0139]FIG. 24 illustrates a projection device 1100 including first, second, third and fourth polarizing beam splitting elements 1102, 1104, 1106, 1108, a first CSP 1110, a second CSP 1112, a third CSP 1114, an optional color filtering element 1116, an optional half wave plate 1118 oriented at 0°, a half wave plate 1120 orientated at 45°, a first reflective panel 1122, a second reflective panel 1124 and a third reflective panel 1126 that operate to modulate a polarized input light beam 1128 to produce an output light beam 1130. The polarized input light beam 1128 may be generated by any suitable light source, including, but not limited to an ultra-high pressure mercury (UHP) lamp, a xenon lamp, a fusion lamp, a light source according to application No. 60/358,517 (which is incorporated herein in its entirety by this reference) or any other suitable light source. The light source also may include suitable projection optics. The polarized input light beam 1128 is incident upon the first CSP 1110 which rotates the polarization such that light that is to be incident upon the first reflective panel 1112 is orthogonally polarized to the light which is to be incident upon the second and third reflective panels 1124, 1126. Depending upon the polarization of the polarized input light beam 1128, this may be achieved by having the CSP 1110 introduce an appropriate amount of polarization rotation at certain wavelengths. The light is then incident upon the first polarizing beam splitting element 1102 which transmits light of one polarization and reflects light of another polarization. The polarizing beam splitting element may be a conventional cube polarizing beam splitter, a Moxtek wire grid polarizer or any other suitable polarizing beam splitting element.

[0140] An optional color filtering element 1116 such as a transmitting dichroic mirror may be included to remove any light outside of the desired spectrum of the reflected light. An optional half wave plate 1118 oriented at 0° may be included to compensate for skew rays as is discussed in herein.

[0141] The reflected light then is incident upon the second polarizing beam splitting element 1104 which reflects the reflected light to form twice-reflected light. The twice-reflected light then is incident upon the first reflective panel 1122 that modulates and reflects the twice-reflected light. The twice-reflected light (the reflections that are being counted are limited to the reflections caused by the polarization beam splitting elements 1102, 1104, 1106, 1108) then travels through the second beam splitting element to half wave plate 1120 oriented at 45°. This quarter wave plate 1120 rotates the polarization of the twice-reflected light to the orthogonal polarization direction such that the fourth polarizing beam splitting element 1108 will reflect the twice-reflected light a third time to form thrice-reflected light. The thrice-reflected light is output from the polarizing beam splitting element 1108 to form part of the output light beam 1130. The output light beam may be projected by any suitable projection optics or may otherwise be imaged for display.

[0142] The light that is transmitted through the first polarizing beam splitting element 1102 is incident upon a second CSP 1112 that rotates part of the remaining spectrum such that the third polarizing beam splitting element 1106 will reflect part of the light to the second reflective panel 1124 and allow the remaining part of the light to be transmitted through the third polarizing beam splitting element 1106 to the third reflective panel. The second reflective panel 1124 modulates and reflects the light back through the third polarizing beam splitting element 1106 to the third CSP 1114. The third CSP 1114 does not rotate polarization of the light reflected from the second reflective panel 1124 such that the light will be transmitted through the fourth polarizing beam splitting element 1108 and become part of the output light beam. The third reflective panel 1126 modulates and reflects the light back into the third polarizing beam splitting element 1106 which reflects the light from the third reflective panel 1126 toward the third CSP 1114. The third CSP 1114 rotates polarization of the light reflected from the third reflective panel 1126 such that the light will be transmitted through the fourth polarizing beam splitting element 1108 and become part of the output light beam.

[0143] The light from the second and third reflective panels 1124, 1126 is reflected once by the third polarizing beam splitting element 1106 and is transmitted through the first, third and fourth polarizing beam splitting elements 1102, 1106, 1108. Conversely, the light from the first reflective panel 1122 is transmitted once by the second polarizing beam splitting element 1104 and is reflected transmitted through the first, second and fourth polarizing beam splitting elements 1102, 1104, 1108. By recognizing that the different frequencies of light have different throughput efficiencies and by recognizing that the polarizing beam splitting elements are more efficient reflectors than transmitters, a projection device 1100 may be design and fabricated. Specifically, the least efficient spectrum of light selected for direction to the first reflective panel 1122 so as to be offset by the higher efficiency of polarizing beam splitting elements. This constraint during design may allow greater design freedom, greater throughput and obviates the need for an output polarizer.

[0144]FIG. 25 illustrates an exemplary embodiment of FIG. 24 where blue light is directed to the first reflective panel 1102, green light is directed to the second reflective panel 1104 and where red light is directed to the third reflective panel 1126. In this example, the first CSP 1110 is a blue/yellow CSP, the optional color filtering element 1116 is a blue color filter, the second CSP 1112 is a green/magenta CSP and the third CSP 1114 is a magenta/green CSP. In practice it is advantageous to make the CSPs 1112, 1114 green/magenta and magenta/green, respectively, to minimize the number of birefringent films for a given spectral transition steepness. However, the CSPs 1112, 1114 of green/red and red/green, respectively, may be used. Alternatively, the second and third reflective panels 1124, 1126 may be switched which changes the second CSP 1112 is a red/cyan CSP and the third CSP 1114 is a cyan/red CSP. Alternatively, any combination of spectra may be used and/or any type of CSPs may be used provided light is directed the desired light paths.

[0145] To avoid 580 nm yellow light leaking from the green and red panels, green/magenta and magenta/green CSPs must not overlap more than about 10% in their spectra with the yellow transition of the green/magenta CSP being at a longer wavelength than the magenta/green CSP. By keeping the overlap to less or equal to about 10%, 580 nm light is substantially absent from the projected spectrum. Thus, saturated green and red colors are achieved without further filtering.

[0146] The yellow cut-off of the green/magenta determines the red color saturation and as such should be about 595 nm when an industry standard UHP lamp is used. The yellow cut-off of the magenta/green filter determines the longest green wavelengths and as such should be close to 570 nm for the UHP lamp. As explained above the slope of these yellow transitions must ensure <10% overlap so 10-90% slopes must be <25 nm. The transition of the blue dichroic mirror should determine spectrum of the blue channel. In the case of the UHP, the transition may be anywhere between 480-520 nm and will have subtle effects on system colorimetry and brightness. In practice, this transition should be short such that the longer blue/yellow transition makes negligible the p-polarized cyan light incident on the blue panel and the blue/yellow transition should not be too long to avoid making the green color too yellow. A good compromise is to have the dichroic cut-off at 495 nm and the blue/yellow to have a 510 nm cut-off. The cyan cut off of the green/magenta CSP and the magenta/green CSPs if shorter than that of the blue/yellow CSP may be ignored. If the cut off is longer than the blue/yellow, the cut off for the green/magenta CSP should be shorter than the cut off for the magenta/green CSP for contrast. Thus, the magenta/green CSP determines the cyan cut-off of the green spectrum and should be 530 nm or less.

[0147] Additionally, skew ray depolarization compensation may be included because contrast is typically given precedence over throughput. For this reason, the CSPs affect on contrast may merit compensation. In the case of the green/magenta CSP sandwiched between polarizing beam splitters, a rotationally invariant design with a half waveplate at 0 degrees optimized for wavelengths 20 nm either side of 550 nm may be used. For magenta/green CSP, a rotationally invariant design and half wave designed to operate in the red is desired. For the blue channel the output polarization rotator should have the reflection symmetry of a retarder making a single half wave plate at 45° a suitable choice. For the input side, a half wave at 0° between the input and blue polarizing beam splitter would negate adverse depolarization effects between these elements and increase blue contrast. For the input CSP, an o-plate would increase the blue contrast although including an o-plate may not be cost effective for systems with an f/#>2.5.

[0148]FIG. 26 illustrates a projection device 200 that uses unpolarized input light 202. The use of dichroic filters 204, 206 at each of the output ports of the first polarizing beam splitting element 1102 allows for the use of the unpolarized input light 202. This embodiment has the advantage of overlapping color spectra. For example, when the first reflective panel 1122 is a blue reflective panel, the p-polarized cyan light may be part of the projected green spectrum and s-polarized cyan may be added to the projected blue light. The number of CSPs is reduced which reduces cost. An optional polarizer 208 may also be included to eliminate reflected p-polarized light.

[0149]FIG. 27 illustrates a projection device 1300 that includes a dichroic input beam splitter 1302. FIG. 27 includes optional additional trim filters 1304, 1306 placed at the output ports of the dichroic input beam splitter 1302 to improve the color saturation. The input light 1308 may be polarized or unpolarized light.

[0150]FIG. 28 illustrates a projection device 1400 with a chromatic polarizing beam splitting element 1402. The projection device 1400 receives unpolarized input light 1404 that is incident upon a graded dichroic 1406. The graded dichroic 1406 divides the light by color into two beams. Those beams are each coupled by a coupling element 1408 to a polarizer 410. The first beam (green light) is reflected by the polarizing beam splitting element 1104 onto the first reflective panel 1124 (e.g., a green panel). The reflective panel 1124 modulates and reflects the light back through the polarizing beam splitting element 1104 and into the chromatic (e.g., red) polarizing beam splitting element 1402 which reflect the light from the first reflective panel 1124 into the projection optics 1412 (e.g., lens).

[0151] The second beam (e.g., red and blue light) has part of its polarization rotated light that is to be incident upon a second reflective panel 1126 (e.g., a red panel) is orthogonal to light that is to be incident upon a third reflective panel 1128 (e.g., a blue panel) by a CSP 414 (e.g., a red/blue CSP). Then a polarizing beam splitting element 1104 reflects part of the light (e.g., red) onto the second reflective panel 1126 and transmits the other part of the light (e.g., blue) to the third panel 1128. The second and third panels 1126, 1128 modulate and reflect the light back to the polarizing beam splitting element 1104. The polarizing beam splitting element 1104 transmits light from the second panel 1126 and reflects light from the third panel 1128. The light from the polarizing beam splitting element 1104 is then incident upon the chromatic polarizing beam splitting element 1402.

[0152] The chromatic polarizing beam splitting element 1402 lets the light from the second reflective panel 1126 be transmitted through the chromatic polarizing beam splitting element 1402 to the projection optic 1412 because the chromatic polarizing beam splitting element 1402 is isotropic to light of that color (e.g., red). For light of any other color, the chromatic polarizing beam splitting element 1402 acts like the other polarizing beam splitting elements 1104, 1106. However, the chromatic polarizing beam splitting element 1402 is oriented orthogonally to the other beam splitting elements 1104, 1106. This orthogonal orientation causes the chromatic polarizing beam splitting element 1402 to reverse the polarizations that are transmitted and reflected. Thus, FIG. 28 eliminates the need for any CSPs between the reflective panels 1122, 1124, 1126 and the chromatic polarizing beam splitting element 1402.

[0153] Alternatively, trim dichroic filters to avoid projecting the unwanted yellow light may also be included in FIG. 28. Lenses that allow the input to be non-telecentric through the dichroic reducing the overall component size may be incorporated into the coupling element 1408. Alternatively, the graded dichroic 1406 may be replaced by a further polarizing beam splitting element and a CSP to improve illumination angular tolerance, with the orientation (0° or 90°) of the input light being selected by the dichroic or CSP performance.

[0154]FIG. 29 illustrates a projection system 1500 with internal polarizers 1502, 1504. The internal polarizers 1502, 1504 let the first panel 1122 be along axis of the output light 1130 and thus along the axis of the projection optics (not shown). FIG. 29 has good phase flatness, allows leakages in the output CSP 1114 without eroding the contrast of the sub-system, negates the need for good pre-polarizing and post analyzing elements, and may have other advantages.

[0155] FIGS. 30-33 illustrate alternative embodiments with internal polarizers 1502, 1504. Alternatively, a chromatic polarizer 1506, 1508 may be used before and/or after the polarizing beam splitting element 1106. A chromatic polarizer 1506 acts as a polarizer for one color band and is isotropic for another. The use of a chromatic polarizer 1506 after a CSP may be used to separate polarization channels. Combinations of crossed chromatic polarizers can also be used, each with specific polarizing and isotropic bands.

[0156] The system 1100 of FIG. 25 provides an efficient means of splitting and recombining color channels while maintaining good polarization and high contrast. Alternatively, FIG. 25 and other embodiments may be configured to achieve very high sequential contrast in systems and/or may be modified to enhance other system performance criteria such as:

[0157] 1. ANSI contrast—by reducing reflection surfaces in the projection path;

[0158] 2. Soft focus—by reducing non-phase flat components and reflecting surfaces in the projection path;

[0159] 3. The sequential contrast of a color—by isolating the channel of the color (e.g., blue);

[0160] 4. Physical compactness—by introducing a 90° angle between input and output transmission axes;

[0161] 5. Brightness at high white color temperatures—by increasing blue transmission; and

[0162] 6. Durability—by removing the output absorbing sheet polarizer.

[0163] The system performance criteria may be improved at the expense of sequential photopic contrast and component specification. Such improvements may be incorporated since PBS cubes with improved transmission of p-polarized light are commercially available and since sequential contrast may not be limiting when compared with off state color and ANSI contrast.

[0164] For example, it may be advantageous to control green and yellow leakage in the OFF-state such that the system 1100 has high sequential contrast in the green. This may be achieved by designing the third polarizing beam splitting element 1106 to reflect as little as possible p-polarized green light and by compensating the third CSP 1114 for skew ray depolarization. To suppress yellow, the third polarizing beam splitting element 1106 may be selected to have a low transmission of s-polarized light (Ts<0.1%@570-590 nm). Optionally, the system 1100 may incorporate a yellow notch (Y/N) filter in or just after the illumination system to filter out the wavelengths between 570 and 600 nm as these wavelengths are not normally utilized in primary color projection systems and their presence complicates design and/or increase the cost of the third polarizing beam splitting element 1106.

[0165] Furthermore, replacing input polarizer with a PBS may increase durability, reduce costs, increase transmission and improve blue/yellow filter performance by allowing skew ray compensation. Another advantage, in cases where light intensity is high, may be obtained by removing the potentially light degrading organic based components from the shorter wavelength light path(s). As the λ/2 filters may be made cost effectively from inorganic quartz, the blue/yellow filter becomes the only polycarbonate filter through which blue light passes. By removing this filter, color separation may be performed with a color selective dichroic. FIG. 34 illustrates a system 1600 having these advantages.

[0166]FIG. 34 includes four PBSs 1602, 1604, 1606 and 1608 and one 45° embedded dichroic beam splitter 1610. The first PBS 1602 is an input pre-polarizing PBS that improves performance by avoiding degradation from birefringence effects in the 45° dichroic coating. This is also advantageous as it has a lower cost and higher performance as compared with durable polarizers.

[0167] Light enters the first PBS 1602 a clean up PBS prior to a 45° embedded dichroic splitter 1610, where p-polarized blue light is reflected and separated from the transmitted red and green light. The blue light is then spectrally cleaned up by a blue transmitting normally incident dichroic 1632 which strips off reflected green and red p-polarized light. A first quartz half-wave plate 1614 then rotates the polarization axes making blue primarily s-polarized and at the same time correcting for skew ray polarization rotation. The s-polarized blue light reflects off the second PBS 1604 and illuminates a blue panel 1616. In the ON-state, this light gets transformed into p-polarized light by the blue panel 1616 and is transmitted though the second PBS 1604. A second quartz plate 1618 rotates the polarization back to s, resulting in reflection of the light by the fourth PBS 1608. This light is then projected by the projection optics 1620.

[0168] The p-polarized green and red light transmitted through the dichroic beam splitter 1610 may have a blue light component depending on the performance of the dichroic coating of the dichroic beam splitter 1610. To avoid contamination of the red spectrum, this blue light is filtered out with the optional, normally incident, yellow transmitting dichroic 1622. The color bands of the transmitted light are then separated by the green/magenta filter 1624, such as a GM ColorSelect™ filter (GMT1) made by ColorLink, into s-polarized green and p-polarized red components so that the third PBS 1606 adjacent the green panel 1626 and red panel 1628 act to separate light to the traveling to the panels 1626, 1628 then combine the reflected light. The combined light exits the third PBS 1606 opposite the green panel 1626 when the panels are ON and a magenta/green filter 1630 such as the MG ColorSelect™ filter (MGT1) from ColorLink acts to rotate the green polarization relative to the red producing yellow p-polarized light. This light is transmitted through the fourth PBS 1608 and is projected together with the s-polarized blue light from the blue panel 1616. The system 1600 may be further improved by including a blue dichroic filter 1632 that filters out non-blue light.

[0169] Unwanted yellow 575-590 nm light is incident on the green panel 1626 since it is treated as green by the input ‘wide-spectrum’ green/magenta filter 1624, whereas the narrower magenta/green filter 1630 treats this wavelength with the longer red wavelengths. As such, this yellow light is s-polarized prior to entering the fourth PBS 1608 and is not projected allowing saturated primary colors to be projected.

[0170]FIG. 34 yields good contrast when green leakage is controlled by the methods described above. Further improvement in contrast generally results if care is taken with regards the polarization integrity in the illumination arm. From a spectral standpoint, the dichroic coating of the dichroic beam splitter 1610 may have good performance for p-polarized light and may have poor performance for s-polarized light. In fact, no specific affect on s-polarized light is required as there should be minimal s-polarized light incident upon it. However low transmission of s-polarization may reduce the specification of the first PBS 1602 with regards to Ts in the green and red parts of the spectrum.

[0171] The dichroic beam splitter 1610 is embedded in a glass cube to reduce system size, lower the incident illumination ray angles and maintain telecentricity through the entire system 1600. Telecentricity is advantageous, in general, to avoid polarization mixing since integral lenses act to depolarize light. Also the embedding of the dichroic in glass matches the geometric polarization axis rotation for skew rays between the successive beamsplitting cubes. This is advantageous because light rays that are not well polarized along either the s or p-polarization axes of a dichroic splitter experience birefringence that degrades polarization and impair system performance. This is the reason for using the first PBS 1602 with a parallel coated surface since the first PBS 1602 acts to polarize each ray correctly and avoids polarization mixing at the dichroic interface. Some glass index mismatch between the prisms embedding the dichroic coating and the glass index of the other prisms in the system 1600 may nonetheless result in minimal or acceptable amounts of skew ray polarization mismatch.

[0172] Embedding a dichroic mirror separates the transition bands for s and p-polarized light. This results in dichroic beam splitter 1610 acting like a PBS for a region of the spectrum. In FIG. 34, this may act to clean up the green and red polarization. Additionally, embedding the dichroic coating in a higher index acts to cut down the ray angles incident on the dichroic coating and reduces the broadening effect incident angles have on the transition slope. This improves the performance of the system 1600. Advantageously, a low elasticity glass material may be use to reduce stress birefringence.

[0173] The light between the transition regions of the green/magenta and magenta/green filters 1624, 1630 may be analyzed by the third PBS 1606. The leakage at these wavelengths, assuming no external filtering, is simply the transmission of s-polarized light of the third PBS 1606. Since the cyan light entering the third PBS 1606 may be controlled by band-pass dichroics, the yellow light between about 575 to about 590 nm will be sufficiently filtered if Ts is low (e.g., the third PBS 1606 has a Ts<0.05%). However, the near conflicting desire of having a high Tp value either side of this spectral region for the third PBS 1606 makes it advantageous to control this yellow light separately. For example, free-standing, low cost, antireflective coated polycarbonate yellow/neutral filters, such as a Y/N ColorSelect™ filter from ColorLink, may be used where polarization conversion illumination systems are utilized without additional polarizers. Alternatively, a yellow/neutral dichroic or a doped absorption glass filter may be used. Alternatively, an external double notch filter, such as a D/N ColorSelect™ filter from ColorLink may be used. An exemplary spectrum for a double notch filter is illustrated in FIG. 35.

[0174]FIG. 36 illustrates a spectra of an exemplary filter set used in FIG. 34. The filter spectra are for parallel polarizer ColorSelect™ filter spectra of the MGT1 and GMT1 ColorSelect™ filters from ColorLink. However, any filter with a suitable filter spectrum may be used.

[0175] The incident angle of light has minimal affect on the performance of normally incident dichroic filters. As such, sharp transitions and good suppression are typical. For example, FIG. 37 illustrates the transmission spectra of the blue and yellow transmitting dichroic plates.

[0176]FIG. 38 illustrates the blue and red transmissions of a FIG. 34 system compared to a FIG. 25 system. For a red starved system, a 20% increase results and for a blue starved system >30% increase results. Furthermore, the system of FIG. 34 has improved ANSI contrast and minimal soft focus issues.

[0177] Other exemplary embodiments similar to the system of FIG. 34 may also be used. For example, the green panel 1626 and the red panel 1628 may be transposed in the system 1600 of FIG. 34 by transposing the green/magenta filter 1624 and the magenta/green filter 1630. This embodiment may allow the use of filters with better characteristics or may have other advantages. Another exemplary embodiment includes a rotationally invariant yellow neutral filter, such as a ColorSelect y/n filter from ColorLink, that utilizes a dichroic beam splitting element having very low Ts in Green and Red. This is advantageous because a polarization conversion system is not needed. Another exemplary embodiment includes a wire grid polarizer, such as made by Moxtek, and an O-plate. Yet another exemplary embodiment includes rotators substituted for the λ/2 waveplates. These rotators maybe multiple birefringent layered filters that act to rotate the polarization, such as the ColorSelect filters available from ColorLink. This is advantageous because their performance can be made more achromatic, and they may prove a less costly component than the quartz plates. Replacing the quartz with a single polycarbonate could also be a lower cost solution, but current manufacturing induced non-uniformities in single layer components are more significant when compared with the multiple layer rotator solutions.

[0178]FIG. 39 illustrates another exemplary system 1700 having a red and green reflecting dichroic beam splitter. The system 1700 of FIG. 39 includes many of the elements of the system 1600 of FIG. 34 which operate substantially the same way. The other elements include an optional yellow/neutral filter 1702 and a red and green dichroic 1704 that reflects the red and green p-polarized light.

[0179] In all the above architectures with internal polarizers, dichroics may be used to avoid p-polarized reflection. Skew ray polarization correction may be used. Finally input polarizing beam splitting elements could utilize dichroic splitters or alternative plate polarizing beam splitting elements. Additionally, sheet polarizers may be used to improve filter performance where an output CSP and polarizer is used.

[0180] The above compensation techniques can compensate skew ray depolarization in most CSPF/PBS architectures and are not limited to color projection systems. This applies equally well to all kinds of beam splitters including but not limited to PBSs, dichroic beam splitters, partially transmissive mirrored surfaces such as a partially silvered mirror beam splitter, and the like. Additionally, these techniques can also be used in any system in which light passes through a series of reflective surfaces at angles other than normal to the propagation direction to maintain polarization integrity.

[0181] The compensating stacks described herein may be made from any suitable material such as solid crystals, stretched polymers, liquid crystal polymers, or another material. The liquid crystal polymer can have dual homogeneous alignment, splay alignment (homogeneous/homeotropic) or any suitable alignment. Although the compensated retarder stacks are discussed in the context of color management for projection display, they can be used in a number of applications. These include, among others, color separation for image capture or radiometry, lighting, and near infrared optical communications.

[0182] Exemplary light sources include ultra-high pressure mercury arc lamps with a small arc size such as is available from Ushio, Toshiba, Perkin-Elmer, Phoenix and others. These light sources are rich in green and blue light but are lacking in red light. Additionally, these light sources produce substantial amounts of UV light which may degrade organic materials such as polycarbonate films. This or any suitable light source may be used to provide light for the systems discussed above. Exemplary embodiments for the yellow double notch filer, input PBS, the dichroic beam splitter, the yellow transmitting dichroic, green magenta filter, the green red PBS, the magenta green filter, the output PBS, the blue transmitting dichroic, the input blue polarization rotating filter, the blue PBS and the output blue polarization filter are listed in the following charts. Property Typical Range Units Comments YELLOW DOUBLE NOTCH FILER Blue 97  95-100 % transmission 430-480 nm Cyan 2 0-5 % Optional transmission 495-505 nm Green 96  94-100 % transmission 520-560 nm Yellow 2 0-5 % transmission 580-590 nm Red 97  95-100 % transmission 610-680 nm INPUT PBS Glass index 1.85 (PBH56) 1.5-2.0 — Size 34 × 34 × 41 N/A mm Average 90 85-95 % Averaged over the transmission of input illumination p-polarized ray set. blue light (Tp) 430-500 nm Green Tp 500- 90 80-95 % 600 nm Red Tp 600- 92 90-98 % 680 nm Average 0.05   0-0.1 % transmission of s-polarized blue light (Ts) 430-500 nm Green Ts 500- 0.1   0-0.2 % 600 nm Red Ts 600- 0.1   0-0.2 % 680 nm External All surfaces may be antireflective coated for surfaces <0.5% reflection over the visible spectrum. DICHROIC BEAM SPLITTER Glass index 1.85 1.5-2.0 — Exemplary glasses include SF2, PBH55, PBH56 or BK7. Size 27 × 27 × 34 N/A mm Average 95  85-100 % Averaged over the reflection of p- input illumination polarized blue ray set. light (Rp) 430- 470 nm Average 95  90-100 % transmission of p-polarized Green light (Tp) 550- 570 nm Red Tp 600- 97  90-100 % 680 nm External All surfaces may be antireflective coated for surfaces <0.5% reflection over the visible spectrum. Free standing is advantageous due to thermal characteristics YELLOW TRANSMITTING DICHROIC Blue 1 0-2 % transmission 430-490 nm Green 96  94-100 % transmission 530-600 nm Red 97  95-100 % transmission 600-680 nm 50% transmis- 515 505-525 nm sion GREEN MAGENTA FILTER Blue Crossed Don't care D/C polarizer - (D/C) transmission 430-480 nm (Bx) Gx (530- 98  97-100 % 560 nm) Rx (615- 1 0-2 % 680 nm) Blue parallel- D/C D/C polarizer transmission (Bp) Gp 0.5   0-0.7 % Rp 98  96-100 % Cyan 495 <500 nm 50% point (where parallel = crossed polarizer transmission. Yellow 570 565-575 nm 50% point General This may be a free-standing part with anti-reflective Comments coatings evaporated directly onto the polycarbonate. GREEN RED PBS Glass index 1.85 (PBH56) 1.5-2.0 — A low coefficient of optical elasticity reduces stress induced birefringence. Size 22 × 22 × 28 N/A mm Blue Tp 430- D/C D/C % 500 nm Green Tp 510- 94 90-97 % 600 nm Red Tp 600- 94 90-98 % 680 nm Blue Ts D/C — % Green Ts 500- 1   0-0.2 % 560 nm Yellow Ts 0.1   0-0.2 % 570-590 nm Red Ts 600- 0.5 0-1 % 680 nm External Input and panels ports may be antireflective coated for surfaces <0.5% reflection over the visible spectrum. MAGNETA GREEN FILTER Blue Crossed Don't care D/C polarizer - (D/C) transmission 430-480 nm (Bx) Gx (530- 1 0-2 % 560 nm) Rx (615- 98 97-99 % 680 nm) Blue parallel- D/C D/C polarizer transmission (Bp) Gp 97 96-99 % Rp 0.25   0-0.5 % Cyan 520 <530 nm 50% point Yellow 570 565-575 nm 50% point General This filter may be bonded between glass substrates with Comments conforming glue, such as silicone, and then the glass is bonded to the adjacent PBSs to avoid reflections. OUTPUT PBS Glass index 1.85 (PBH56) 1.5-2.0 — A low coefficient of optical elasticity reduces stress induced birefringence. Size 27 × 27 × 34 N/A mm Blue Tp 430- D/C D/C % 500 nm Green Tp 500- 94 90-97 % 600 nm Red Tp 600- 95 90-98 % 680 nm Blue Ts 0.1%   0-0.2 % Yellow Ts D/C 500-680 nm External All surfaces may be antireflective coated for surfaces <0.5% reflection over the visible spectrum. BLUE TRANSMITTING DICHROIC Blue 97 96-99 % transmission 430-480 nm Yellow 0.5 0-1 % transmission 510-680 nm 50% transmis- 485 480-490 nm sion INPUT BLUE POLARIZATION ROTATING FILTER (QUARTZ 45° λ/2) Blue Parallel 1 <1.5 % Transmission at polarizer - 450 nm is <0.5%. transmission 430-470 nm (Bp) Yx and Yp D/C (510-680) Blue crossed- 97 >95 % polarizer transmission (Bx) BLUE PBS Glass index 1.85 1.5-2.0 — A low coefficient (e.g., PBH56) of optical elasticity reduces stress induced birefringence. Size 22 × 22 × 28 N/A mm Blue Tp 430- 95 >93 % 520 nm Yellow Tp D/C 520-680 nm Blue Ts 0.3 <0.5 % Yellow Ts D/C 520-680 nm External Input and panels ports may be antireflective coated for surfaces <0.5% reflection over the visible spectrum. OUTPUT BLUE POLARIZATION ROTATING FILTER (QUARTZ 45° λ/2) Blue parallel 1 <2 % polarizer - transmission 430-470 nm (Bp) Blue crossed- 97 >95 % polarizer transmission (Bx) Yx and Yp D/C (510-680)

[0183] Although the exemplary embodiments described herein may be described in reference to specific colors or combinations of colors, it should be understood that other color combinations are also possible. For example, the color filters can be any combination of colors including Red/Blue, Blue/Green, Red/Green, an additive primary and its complementary subtractive primary or any other color pair as system criteria dictate.

[0184] As used herein, rotation is not limited to a pure optical rotation but also includes any transformation that results in optical rotation or other effects that result in optical rotation. For example, a half wave plate at 45° can transform a linear state into an orthogonal linear state via retardation.

[0185] Although several embodiments of the present invention and its advantages have been described in detail, it should be understood that changes, substitutions, transformations, modifications, variations, permutations and alterations may be made therein without departing from the teachings of the present invention, the spirit and the scope of the invention being set forth by the appended claims. 

We claim:
 1. An optical filter comprising: at least three retarders, the at least three retarders causing optical rotation to light of a first spectrum substantially without introducing retardation.
 2. The filter of claim 1, wherein the at least three retarders are isotropic to light of a second spectrum.
 3. The filter of claim 2, further comprising: a bias retarder, wherein the bias retarder and the at least three retarders have about a half wave of retardation.
 4. The filter of claim 3, wherein the filter is an achromatic half wave retarder in the first spectrum and in the second spectrum.
 5. The filter of claim 3, wherein the filter has a substantially wavelength stable eigenpolarization.
 6. The filter of claim 1, wherein the optical rotation is a 90° optical rotation.
 7. The filter of claim 1, wherein the optical rotation of the at least three retarders is achromatic in the first spectrum.
 8. The filter of claim 1, wherein the at least three retarders have a substantially wavelength stable eigenpolarization.
 9. The filter of claim 1, further comprising a first beam splitter in optical series with the at least three retarders.
 10. The filter of claim 9, wherein the first beam splitter is a polarizing beam splitter.
 11. The filter of claim 9, wherein the first beam splitter is a dichroic beam splitter.
 12. The filter of claim 9, wherein the first beam splitter is a partially metallized mirror beam splitter.
 13. The filter of claim 9, further comprising a second beam splitter in optical series with the at least three retarders and the first beam splitter.
 14. The filter of claim 1, further comprising: a first beam splitter and a second beam splitter, wherein the at least three retarders are between the first beam splitter and the second beam splitter; and wherein skew light ray polarization effects of the first beam splitter are offset by skew light ray polarization effects of the at least three retarders and the second beam splitter.
 15. The filter of claim 14, wherein the first beam splitter and the second beam splitter have a common normal vector.
 16. The filter of claim 1, wherein the at least three retarders further includes a bias retarder to make a retardation of the at least three retarders have substantially no retardation.
 17. An optical filter comprising: at least three retarders, wherein the at least three retarders act as a half wave plate to light of a first spectrum; and wherein the at least three retarders act as a half wave plate with a π/4 displaced optical axis to light of a second spectrum.
 18. The optical filter of claim 17, wherein the at least three retarders act as a half wave plate with an orientation of OE in the first spectrum and 45E in the second spectrum.
 19. The filter of claim 17, wherein the at least three retarders have wavelength stable half wave retardation and wavelength stable eigenpolarization in at least one of the first spectrum and the second spectrum.
 20. The filter of claim 17, wherein the at least three retarders are achromatic in the first spectrum and the second spectrum.
 21. The filter of claim 17, further comprising a first beam splitter in optical series with the at least three retarders.
 22. The filter of claim 21, wherein the first beam splitter is a polarizing beam splitter.
 23. The filter of claim 21, wherein the first beam splitter is a dichroic beam splitter.
 24. The filter of claim 21, wherein the first beam splitter is a partially metallized mirror beam splitter.
 25. The filter of claim 21, further comprising a second beam splitter in optical series with the at least three retarders and the first beam splitter.
 26. The filter of claim 25, wherein the first beam splitter and the second beam splitter have an orthogonal normal vector.
 27. The filter of claim 17, further comprising: a first beam splitter and a second beam splitter, wherein the at least three retarders are between the first beam splitter and the second beam splitter; and wherein skew light ray polarization effects of the first beam splitter are offset by skew light ray polarization effects of the at least three retarders and second beam splitter.
 28. The filter of claim 17, further comprising a color projection system.
 29. The filter of claim 17, wherein the at least three retarders are an out-of-plane uniaxial compensator.
 30. The filter of claim 29, wherein the at least three retarders have an optical axis θ between about 26.0° to 26.5°.
 31. The filter of claim 17, wherein the at least three retarders have a substantially wavelength stable eigenpolarization along a direction of a design axis of the at least three retarders.
 32. An optical filtering method, the steps of the method comprising: optically rotating light of a first spectrum without introducing retardation.
 33. The method of claim 32, wherein the optically rotating is performed by three or more retarders.
 34. The method of claim 33, wherein the three or more retarders are achromatic in the first spectrum.
 35. The method of claim 33, wherein the three or more retarders have a substantially wavelength stable eigenpolarization.
 36. The method of claim 32, further comprising separating light into two different paths.
 37. The method of claim 36, wherein the separating is according to polarization.
 38. The method of claim 36, wherein the separating is according to light wavelength.
 39. The method of claim 31, further comprising: a first separating of light into two different paths; a second separating of light into two different paths; wherein the optically rotating occurs after the first separating but before the second separating such the optical rotating is substantially independent of skew ray direction.
 40. The method of claim 31, further comprising transmitting light of a second spectrum unaltered.
 41. The method of claim 31, wherein the optical rotating is a 90° optical rotation.
 42. An optical filter method, the steps of the method comprising: retarding light of a first spectrum; and retarding light of a second spectrum; wherein a half wave of retardation is provided by the retarding light of the first spectrum; and wherein a retardation equivalent to a half wave plate with a π/4 displaced optical axis is provided by the retarding light of the second spectrum.
 43. The method of claim 42, wherein the retarding light of the first spectrum and the retarding light of the second spectrum are achromatic.
 44. The method of claim 42, further comprising separating light into two different paths.
 45. The method of claim 44, wherein the separating is according to polarization.
 46. The method of claim 44, wherein the separating is according to light wavelength.
 47. The method of claim 42, further comprising: a first separating of light into a first path and a second path; a second separating of light into a third path and a fourth path; wherein the optical retardation occurs after the first separating but before the second separating such that skew light ray polarization effects of the first separating are compensated by the optical retardation so as to match skew light ray polarization effects of the second separating.
 48. The filter of claim 47, wherein the first path and the third path are parallel; and the second path and fourth path are antiparallel.
 49. The method of claim 42, wherein the retarding light of the first spectrum and the retarding light of the second spectrum have substantially wavelength stable eigenpolarizations.
 50. An optical arrangement comprising: a half waveplate; and a pair of beam splitters oriented orthogonally to each other, the pair of beam splitters sandwiching the half waveplate.
 51. The arrangement of claim 50, wherein the pair of beam splitters are a pair of polarizing beam splitters.
 52. A method of filtering light, the steps of the method comprising: splitting a first light beam to form a second light beam; retarding the second light beam with a half wave of retardation to form a third light beam; and splitting the third light beam.
 53. The method of claim 52, wherein the splitting the first light beam and the splitting the second light beam are polarized splittings.
 54. An optical arrangement comprising: a planar polarizer; a beam splitter; and an out-of-plane retarder between the planar polarizer and the beam splitter.
 55. The arrangement of claim 54, wherein the out-of-plane retarder is a color selective polarizing filter.
 56. The arrangement of claim 54, wherein the planar polarizer has a transmission axis parallel or perpendicular to a plane containing an optic axis of the out-of-plane retarder.
 57. T he arrangement of claim 54, wherein the beam splitter is a polarizing beam splitter.
 58. A method of filtering light, the steps of the method comprising: polarizing an incident light beam to from a polarized light beam; retarding the polarized light beam with an out-of-plane retarder to form a retarded light beam; and splitting the retarded light beam.
 59. The method of claim 58, wherein the out-of-plane retarder is a color selective polarizing filter.
 60. A color management architecture comprising: a first reflective panel that modulates light of a first spectrum; a second reflective panel that modulates light of a second spectrum; a third reflective panel the modulates light of a third spectrum; and at least four polarizing beam splitting elements, wherein the light of the first and second spectrum is transmitted through more of the at least four polarizing beam splitting elements than the light of the third spectrum, wherein the light source intensity or throughput efficiency of light of the third spectrum being a lower than the light source intensity or throughput efficiency of light of the first or second spectrums when throughput absent the losses associated with transmission or reflection through the polarizing beam splitting elements are excluded.
 61. The architecture of claim 60, wherein light that is output from the at least four polarizing beam splitting elements is not incident upon an output analyzer.
 62. A method of providing a color management system comprising: modulating a first spectrum of light with a first reflective panel; modulating a second spectrum of light with a second reflective panel; and modulating a third spectrum of light with a third reflective panel, wherein the light of the first and second spectrum is transmitted through more of at least four polarizing beam splitting elements than the light of the third spectrum, and wherein the throughput efficiency of light of the third spectrum being lower than the throughput efficiency of light of the first or second spectrums when throughput absent the losses associated with transmission or reflection through the polarizing beam splitting elements is excluded.
 63. A color management architecture comprising: a first reflective panel that modulates a first spectrum of light; a second reflective panel that modulates a second spectrum of light adjacent a first polarizing beam splitting element; a third reflective panel that modulates a third spectrum of light adjacent a second polarizing beam splitting element; and a chromatic polarizing beam splitting element, wherein the first and second polarizing beam splitting elements reflecting light of a first polarization and transmitting light orthogonal to the first polarization; and wherein the chromatic polarizing beam splitting element transmitting light of a first polarization of the first spectrum and reflecting light orthogonal to the first polarization.
 64. The architecture of claim 63, further comprising the absence of a polarization rotation element between the chromatic polarizing beam splitting element.
 65. The architecture of claim 63, wherein the first polarizing beam splitting element, polarizing beam splitting element and the chromatic polarizing beam splitting element having reflective surfaces oriented orthogonal to each other.
 66. A display architecture comprising: an input polarizing beam splitter; a dichroic beam splitter that receives light including light of a first spectrum, a second spectrum and a third spectrum from the input polarizing beam splitter; a first polarizing beam splitter that receives light including the first spectrum from the dichroic beam splitter; a second polarizing beam splitter that receives light including the second spectrum and third spectrum from the dichroic beam splitter; a third polarizing beam splitter that receives light including the second spectrum and third spectrum from the dichroic beam splitter; a first panel for modulating light of the first spectrum adjacent the first polarizing beam splitter; a second panel for modulating light of the second spectrum adjacent the second polarizing beam splitter; and a third panel for modulating light of the third spectrum adjacent the second polarizing beam splitter, wherein a reflective surface of the dichroic beam splitter is optically parallel to a reflective surface of the input polarizing beam splitter.
 67. The architecture of claim 66, further comprising: a yellow/neutral filter, the input polarizing beam splitter being optically sandwiched between the dichroic beam splitter and the yellow/neutral filter.
 68. The architecture of claim 66, further comprising: a first dichroic filter that transmits light of the first spectrum optically between the dichroic beam splitter and the first polarizing beam splitter; and a second dichroic filter that transmits light of the second spectrum and the third spectrum optically between the dichroic beam splitter and the second polarizing beam splitter.
 69. The architecture of claim 66, further comprising: a first inorganic λ/2 plate optically between the dichroic beam splitter and the dichroic beam splitter; and a second inorganic λ/2 plate optically between the first polarizing beam splitter and the third polarizing beam splitter.
 70. The architecture of claim 69, wherein the first inorganic plate and the second inorganic plate are made of quartz.
 71. The architecture of claim 68, further comprising: a first polarization rotation filter that rotates the second spectrum of light but does not substantially rotate the third spectrum of light; and a second polarization rotation filter that rotates the third spectrum of light but does not substantially rotate the second spectrum of light.
 72. The architecture of claim 69, wherein the first polarization rotation filter and the second polarization rotation filter are retarder stacks.
 73. A display architecture comprising: an input polarizing beam splitter; a dichroic beam splitter that receives light including light of a first spectrum, a second spectrum and a third spectrum from the input polarizing beam splitter; a first polarizing beam splitter that receives light including the first spectrum from the dichroic beam splitter; a second polarizing beam splitter that receives light including the second spectrum and third spectrum from the dichroic beam splitter; a third polarizing beam splitter that receives light including the second spectrum and third spectrum from the dichroic beam splitter; a first panel for modulating light of the first spectrum adjacent the first polarizing beam splitter; a second panel for modulating light of the second spectrum adjacent the second polarizing beam splitter; a third panel for modulating light of the third spectrum adjacent the second polarizing beam splitter; a first dichroic filter that transmits light of the first spectrum optically between the dichroic beam splitter and the first polarizing beam splitter; a second dichroic filter that transmits light of the second spectrum and the third spectrum optically between the dichroic beam splitter and the second polarizing beam splitter; a first inorganic λ/2 plate optically between the dichroic beam splitter and the dichroic beam splitter; a second inorganic λ/2 plate optically between the first polarizing beam splitter and the third polarizing beam splitter; a first polarization rotation filter that rotates the second spectrum of light but does not substantially rotate the third spectrum of light; and a second polarization rotation filter that rotates the third spectrum of light but does not substantially rotate the second spectrum of light, wherein a reflective surface of the dichroic beam splitter is optically parallel to a reflective surface of the input polarizing beam splitter.
 74. The architecture of claim 73, further comprising: a yellow/neutral filter, the input polarizing beam splitter being optically sandwiched between the dichroic beam splitter and the yellow/neutral filter.
 75. A display architecture comprising: an input wire grid polarizer optically followed by an o-plate; a dichroic beam splitter that receives light including light of a first spectrum, a second spectrum and a third spectrum from the input wire grid polarizer and the o-plate; a first polarizing beam splitter that receives light including the first spectrum from the dichroic beam splitter; a second polarizing beam splitter that receives light including the second spectrum and third spectrum from the dichroic beam splitter; a third polarizing beam splitter that receives light including the second spectrum and third spectrum from the dichroic beam splitter; a first panel for modulating light of the first spectrum adjacent the first polarizing beam splitter; a second panel for modulating light of the second spectrum adjacent the second polarizing beam splitter; a third panel for modulating light of the third spectrum adjacent the second polarizing beam splitter; a first dichroic filter that transmits light of the first spectrum optically between the dichroic beam splitter and the first polarizing beam splitter; a second dichroic filter that transmits light of the second spectrum and the third spectrum optically between the dichroic beam splitter and the second polarizing beam splitter; a first inorganic λ/2 plate optically between the dichroic beam splitter and the dichroic beam splitter; a second inorganic λ/2 plate optically between the first polarizing beam splitter and the third polarizing beam splitter; a first polarization rotation filter that rotates the second spectrum of light but does not substantially rotate the third spectrum of light; and a second polarization rotation filter that rotates the third spectrum of light but does not substantially rotate the second spectrum of light.
 76. A method of projecting light comprising: polarizing light with an input polarizing beam splitter; splitting light received from the input polarizing beam splitter into a first portion and a second portion with a dichroic beam splitter having a reflective surface optically parallel to a reflective surface of the input polarizing beam splitter; directing the first portion of light to a first panel with a first polarizing beam splitter, the first panel modulating and reflecting the first portion of light; splitting the second portion of light into a third portion of light and a fourth portion of light with a second polarizing beam splitter, the third portion of light being directed to a second panel and the fourth portion of light being directed to a third panel, the second panel modulating and reflecting the third portion of light, the third panel modulating and reflecting the fourth portion of light; combining modulated and reflected third and portions of light with the second polarizing beam splitter to form a fifth portion of light; and combining modulated and reflected first portion light with the fifth portion of light with a third polarizing beam splitter. 